Because it creates instability in someone's life and easily causes folks to work for hours just to have no paycheck if they didn't sell a car (for example). Another example is a cleaning service: I knew someone that worked for one. She got paid a percentage of the cost - she had a schedule, but folks cancelled and other such things. It wound up that she was stuck at work in between jobs, couldn't go home, yet wasn't getting paid.

In other words, it creates a job environment that easily exploits people.

I'm not absolutely offended by bonuses and commissions, mind you, but base pay realistically should make up the majority to most of the salary and be fairly consistent from week to week. Pay based on tips can do the same thing (and you can cause this to happen to tipped employees with scheduling).