Main reason is advertising. On third party app we don’t see ads. I used Apollo and there where no adds. Also there was to ticktok scrolling stuff like in official app. I did not use non official YouTube apps but I think it’s similar case. Also if you use there apps they can serve you content in a way that is more engaging like instagram is to get you even more addicted to the platform. Hope I explained it well, let me know if you have any other questions. 🙂