Why is it so difficult for trading companies to simply keep the money in the reserve and never touch it unless your client wants to liquidate his share?

I know, having that huge pile of cash and not doing anything with it can lead to huuuuuge temptations, but that's what I'm paying the company to do. If I wanted to invest my funds into something I would move my money to a separate investment account that the institution can play with as they please (with some client-defined risk restrictions) and give me a percentage of profits