Simply because in 14, we don't fly from one end of the world to the other like in PWI, and teleporting isn't a bish like PWI either. If SE decides to make the whole of Eorzea into one huge open world map, then I guess upgrading mount speed might be viable, otherwise, I don't see a need to.

Also, the mounts in 14 come with one speed. PWI fliers start you off at default 2? Can't remember, been too long, unless you cash shop the speedier ones.