I think I'd prefer riding the Seahorses as opposed to turning into one for a race. What I thought was really neat about the 70's cartoon is that both Aquaman and Aqualad rode the Seahorses.
If they did that then they would be admitting that they had the tech to make mounts. Then everybody and their grandmas would request custom mounts.
Seems to me like it might be Tech issues. Like we still can't ride cars as well, and there are cars in the game. So yeah...
Also, you don't ride the gorilla, you turn into one for that race. I'd like to ride a gorilla despite how bizarre that would be.