I want to be a YouTuber or in a band but people want me to get good grades and go into a "proper job" but I think I'm entitled to do whatever I want and I want to do that. Why do they want me to get a proper job?

Because youtube is not a job. I personally find it appalling that people view it as a legitimate career path. If you are a talented musician, there is no reason why you can't be successful in a band. That's a much better choice. Good luck :)