They certainly do pocket the difference. But the point is the behavior we see is that what we would see if Steam wasn't enforcing uniform pricing. Publishers would pick some list price on other platforms / their own sites that undercuts Steam and also gives higher margin for themselves.

E.g. a $60 game on Steam with a 30% cut nets you ~$42. If you list the game on for $50 on Epic with a 12% cut, you net $44. The price differential works in favor of the consumer and publisher, and would convince more people to buy on the high-margin platform. A greedy publisher isn't going to keep the $60 price tag, because that just pushes consumers to buy on Steam (which has the most features).