As I see it, 'legitimate career' can be anything that provides you, pays the bills and gets food on your table. As long as you get money out of it, it can be seen as a work/job/career. There are people who make a living by taking selfies(!?) or making videos of how they unbox things(!!??). It's weird, but apparently there are market for entertainment like this... As in many cases, "hate the game, not the player". I personally value "real" old school professions like teacher, factory worker, cleaner etc. more than youtube critics and such, but if someone can turn it into a profitable business, that's cool by me.
In some cases, youtube content creators can be like entrepreneurs or remote workers. They don't necessarily have a traditional nine-to-five jobs, but that doesn't mean that some of them wouldn't dedicate huge amount of time and care into their videos. Not all of them, of course...
It's also worth remembering that artists, writers, musicians, athletics and the rest can equally be seen as similar freeloaders. They don't necessarily produce any physical goods that other people need, but I guess that's one of the definitions for entertainment and art...