As more agents enter the business, a lot of their time and effort ends up going into getting clients in the first place. You end up with lots of agents spending more of their time trying to get business, with actual home-sales work taking up a smaller proportion of their time. But the increase in home prices is enough to make up for the decrease in the number of homes sold per agent.

Point being, an increase in home prices, and the resulting increase in commissions (in dollar terms) doesn't necessarily result in any benefit to clients.