That's just a business problem, not any different for domain names than anything else. I have a great idea for a business, it's a small building that sells pizza. Unfortunately I can't name it Pizza Hut because that's already taken. I want to start a supermarket and my last name is Wall, I can't exactly call it Wallmart. I want to start an electronics supply company, I can't call it Tesla. I have to come up with other names.
I want to create an app that helps people relax. Sorry, Calm is taken, I need a different name. I want an app that's like a book of faces... Facebook is taken. I need a different name.
If someone else already has the domain name you want, you can register under a different tld, but you have to be aware that this conflict will exist and confusion will be a problem.
