IMO, I think this is a valid question (though asked a bit crudely). To generalize, it's because someone who has done JavaScript programming can point to NodeJS and say, "I'm full stack." Which, while technically true, skates over the reality of NodeJS as a less than ideal backend (see Ryan Dahl and the dawn of Deno). Second, it's more lucrative to bill oneself as "full stack" even if in reality a person isn't. At one point in my career I would have considered myself "full stack". Over time I realized that a "full stack" engineer is a jack-of-all-trades type, which in theory, can be valuable in the right circumstances as an individual doing work alongside non-full-stack engineers. Asking a room full of "full stack" engineers to design and build a product of any complexity above CRUD will naturally lead to a self-sorting of UI/UX and server side engineers.