Not everybody is happy with the results. Experts don’t always have your best interests at heart, nor do things that you feel are what you want done to your material.
Some people just want to be able to do it themselves, to expand their capabilities, or to be masters of their own destiny. Whatever floats one’s boat.
At any rate, one has to appreciate what mastering involves, just so one knows what to expect, and perhaps make better mixes, which will save mastering money AND will produce a better result regardless. Throwing it ‘over the fence’, as anyone in IT knows, usually produces something other than what you expect.