Because the animations are designed with the current speed, and increasing the speed will make some, if not all mounts look incredibly ridiculous. Picture a Behemoth running 2-3 times as fast as it does now, or hell even a Goobbue.

They could make varying mounts have different speeds but that comes with its own cons, people might not be able to use their favourite mounts anymore because it's slower, and then they'll complain about their mounts not being able to run ridiculously fast. To give you an example from WoW, this turtle.