The problem with Icarian Flight is it doesn't provide a practical solution when you land... Tarhiel was a glaring example of that miscalculation.
Here we go. This irrational explanation yet again. Arena and Daggerfall are far larger than any RPG ever made, yet they didn't offer mounts to my knowledge. Skyrim and Oblivion are still absolutely huge, yet you try and use other open worlds as reasons to rationalize no mounts? That doesn't make any sense. One being bigger or smaller is irrelevant, as they are ALL big.
Wonky havok physics is hardly an excuse to not add something. There are plenty of mods that add vehicles and even vertibirds that players can control in Fallout 3 and Fallout NV. They work fine and the map is not too small to accommodate them.
You are the most unlucky person in the world. I'd be scared to even go outside if I were you.
I don't see how BGS' ineptitude with horses has anything to do with developing a motorcycle or a car. Again, I fail to understand why BGS shouldn't do something that makes sense and improves the game.