Professors are the producers and consumers of these journals. So, a group of your peers invests time reviewing your work and giving you helpful feedback, and you pay them back by reviewing other people's work.

Basically, it's a zero sum game. So if everybody paid for the reviews they did and then got paid for doing reviews, a lot of money would change hands and everybody would end up with the same amount of money they had to start with.

Of course, the companies that _publish_ the journals take a cut. But that's what it is, a cut. There are really communities of professors participating in a zero sum game.