I’ll list few reasons;
Have you tried trading a large size from home alone?
Have you tried trading a large size from a trading desk with people around you?
If so, then you’ll notice that there is lots of accountability when you have a boss (head trader) and other traders around you, and therefore for many people it is much easier to trade larger size for a prop firm than from a home office in terms of accountability and psychological support.
I’m not saying that ‘s the case with Topstep, I’ve never dealt with them, but there are many benefits from trading for a decent firm.
There is no need to be worry about others knowing one’s trading strategies. For example in FX prop firms one trader can be very technical, anotherone can be very good at trading news. Each trader has his own method that fits him.
Paying percentage from profits is not an issue at all because you have access to large capital, it can be a win/win situation for both parties.
That's not how these kind of prop firms work ...