Not immersion, all populations that have a chance to domesticate an animal useful for transport did it (horses, camels, dromedaries).
The problem in some MMORPGS is that they started to use as mounts whatever the players asked for, giant insects, big cats, reindeers, big reptiles and so on.. ...
I personally don’t like fancy mounts even in a fantasy world. Those should be very exceptional so I can agree with you.
But...in Eberron, halflings ride dinosaurs. It seems the general direction of mounts in games, including tabletop, is to add exotic types. I don’t like the trend but it’s here.