Sales people can vary a lot in the amount of money they bring in, and the variance isn't necessarily explicable by things in their CV. In other words a guy can talk a good talk as well as have a positive record, but when you drop them into your org, you're taking a chance on whether they manage to get anything done. (Which is true for devs as well, but people tend to think -wrongly- they know how to identify devs.)
So paying them on commission is necessary because if they fail you want them out and you cannot afford to pay them anyway. If they do well they share in some of the upside from the uncertainty crystallising to positive.
Sales people can vary a lot in the amount of money they bring in, and the variance isn't necessarily explicable by things in their CV. In other words a guy can talk a good talk as well as have a positive record, but when you drop them into your org, you're taking a chance on whether they manage to get anything done. (Which is true for devs as well, but people tend to think -wrongly- they know how to identify devs.)
So paying them on commission is necessary because if they fail you want them out and you cannot afford to pay them anyway. If they do well they share in some of the upside from the uncertainty crystallising to positive.