A big point is: (1) As an employee X, the employer knows how much X is making and will try to keep that down, while (2) a person X who owns their own business that happens to be a good business can just rake in the money with nearly no one else the wiser who can stop or slow X.

E.g., while I was growing up, a guy in the neighborhood was doing really well. He was in the peanut vending business, you remember, put in a coin, turn a crank, and get out some peanuts. So the customer, how do they know how much money the peanut vendor is making?