They are basically different languages (calling them dialects of one Chinese language is really more a political thing -- a bit like calling the languages of Europe dialects of Latin).
Born in a Hakka family, growing up in Cantonese-speaking region, having studied in places where ~50% of the students are Teochew people, and working with people who speak Wenzhounese, I consider Cantonese, Hakka language, Teochew, Wenzhounese, Mandarin Chinese all dialects of the Chinese language, and can't see why it is so special about Cantonese. At least Wenzhounese is much more different from Mandarin Chinese than Cantonese is, but why use the deviation from Mandarin Chinese as a standard anyway? Mandarin Chinese itself should be a dialect.
To be counted as a different language, I would say it should be as different as Zhuang language, Tibetan, Uyghur, Manchu, Hmong, etc. People who speak these languages might or might not come from politically controversial regions, but I don't think anyone would consider these languages Chinese for a second.
They share the majority of the syntactic and grammar. Their written form are all the same. Pronunciation is the major difference. This level of unity is established probably since the Qin dynasty.
Easiest way to see this difference is watch the news in Cantonese -- it is mostly 书面语 (standard) mandarin but pronounced as Cantonese, whereas day-to-day spoken Cantonese is completely different in structure and vocabulary. What do you think gets written in HK movie scripts? Definitely not standard written mandarin.
Your assertion is a political one, and not universally shared. The Central Party has incentives to make that claim for nationalist reasons, and has been doing so for decades, but that does not necessarily make it true. In my personal opinion, whether it is a dialect or not should be answered by the people who speak it, not by the people who rule them.
Well, you sort of prove my point, that it's a political thing, about the unity of the country.
Here's what linguists say:
1. From "Visible Speech: The Diverse Oneness of Writing Systems" by John DeFrancis:
"Chinese [...] is an umbrella designation for at least eight present-day varieties of what are usually called "dialects" but, since they are mutually unintelligible, might better be considered parallel to the various languages that make up the Romance group of languages."
2. From "Asia's Orthographic Dilemma" by Wm. C. Hannas:
"some eighty million or more people living in China [...] speak non-Chinese languages written in alphabetic or indigenous systems. [...] If we ignore this inconvenient phenomenon and focus on the speech of China's Han population, we find a collection of at least seven or eight mutually unintelligible varieties that in any other context would be called "languages," but which are "dialects" in China, in part for political reasons and in part because of a problem with the translation of the Chinese term fāngyán. The political motivation for claiming that these distinct varieties constitute a single language is fairly obvious: it is easier to govern a country in which the majority believe they are speaking one "language" (whatever the linguistic reality) composed of several "dialects" instead of several related languages.
[...]
Most linguists familiar with the classification problem acknowledge that the major Chinese varieties differ from each other at least on the order of the different languages of the Romance family.
[..]
We have seen that the Chinese languages differ not just in pronunciation but also in vocabulary and grammar, and that these differences are realized through unique morphemes (or unique uses of shared morphemes) for which characters do not exist at all, do not exist in Mandarin, or are used with different meanings and functions. Consequently, character texts in Cantonese and (where available) in Taiwanese are largely unintelligible to Mandarin readers. Many characters are completely unfamiliar; others are recognizable but make no sense in context. This occurs where conventions exist for writing the non-Mandarin variety in characters. Actually, most of these languages have no established writing system and hence lack even the possibility of being understood by readers of other varieties.
This is also true of Spanish and Portuguese but they are still considered different languages. If two people speaking different dailects couldn't understand each other they are surely different languages
To take a different perspective, calling them separate languages helps reinforce a nationalist agenda. It gives your citizenry another reason to feel separate from, and maybe superior to, other nations. Calling Spanish and Portuguese separate languages, instead of two Iberian dialects of modern Latin, helps reinforce the idea that Spain and Portugal are separate nations, and not just different regions of the same peninsula.
It's not really by default, more like by necessity. Almost all categorization schemes for human cultural artifacts are necessarily blurry at the edges, and just because someone has provided a definition that is useful for one purpose (the study of linguistic differences across cultures) does not mean that it is suited for another purpose (rhetorically implying solidarity or separation between two sets of people).
To be consistent with the linguistic facts, one should speak of the Chinese languages. You could also consistently speak of a Chinese language with dialects, and then also consider Romanian, French, Spanish, Portuguese different dialects of the Romance language; but that would redefine the terms from how they're currently commonly understood.
It is a subject of argument because there is no clear definition what is a language. There is spoken language and there is written language, then there is culture behind the language. Chinese dialects resemble complete different spoken languages. However, they all share the same written language. Chinese were rarely written down exactly as the way it was spoken, and this is being practiced since two thousand years ago (probably ever since the unification of the First Emperor). In fact, because of this written language, it reflects back to spoken language as well. Chinese may speak the way they write -- that is, they may speak the same way as their written form, with their various dialect pronunciations. In a way, there is spoken Chinese for the illiterate people, and there is spoken Chinese for literate people. Last, with a common written language, Chinese shares a culture. A language communicates meaning and the meaning is the meaning of the culture. With the same culture, there is little barrier communicating (even between people who speak different dialect). It is weird, but it is not unusual to have two people communicate in two different Chinese dialect -- won't that be the practical definition of a language?
And by politics, I think one really means culture. It is the need of communicating with each other on a daily basis forges a language.
Well, I was mostly commenting on the traditional Chinese. Traditionally most population are illiterate so there is little problem having disconnected spoken language and written language. With the modernization and majority of population becoming literate, there is a unification between spoken language and written language. That has updated both Chinese spoken language and written language to a common form -- dialects are becoming merely a different pronunciation form. Today, with schools mandate speaking of mandarin, dialects (the spoken languages) are on the way out.
This process take place in both the main land and Hong Kong and Taiwan. But due to the isolation, they took slightly different form. That is how to a foreigner's view, Chinese dialects appears to be different languages. This is not fundamentally different from between French and Spanish, only the extent of time differs. With only decades, the Chinese in Hong Kong and main land, e.g., are still viewed by most Chinese as the same language.
