Charlie: because a fixed match cannot be run properly without the crowd catching on early with your idea. The way they do it, the suckers who paid premimum prices for the seats gets to think their guy is ahead until the decision is announced. Also, this would force the judges to deal with incredible hostility from the crowd all match long instead of just at the end, and the side who is getting screwed would be yelling at them, and they'd get reamed all fight long on HBO. It's far easier to tell a match is fixed if you can see it round by round and know beyond doubt it was fixed.
I gave up on boxing long ago because, and I am serious about this, a lot of the key matches leading to championships and some of those stunk to high heaven.