Because if you're a 18 year old central asian you probably don't have the money to buy a pc good enough to stream with and you probably live with your parents so you need a place to work. And you also don't even know where to get started such as what sites to use, toys to buy, just generally how to operate a show and stream. You might also not speak english so you can't communicate with your target audience. An agency takes care of all of that. Why they don't move once they've learned how it works is usually because an agency usually controls the account.
If a model says they're quitting and then show up on a new account a bit later then that usually means they left the agency so they had to start a new account. If you know harooniks / juliastapes she did that. Hee_jeen did it recently too. Another model lemmexy said she had told her agency she wanted to go off on her own and they agreed to give her her accounts but then after she left they deleted her accounts.
This part is just speculation but I assume working with an agency gives a sense of security and legitimacy too. It can probably feel sketchy doing it on your own since its probably illegal or in a gray area in alot of countries. So at least with an agency its not just you in it alone.