You could do that, Proweler, and then whenever you were close to a city, everything associated with the city would load and slow the game down -- even if you had no intention of entering the city itself. But be my guest.
You can't fly, climb or jump over the city walls, Ghanburighan, so that's not an issue, but you can see landscape and such that's outside the walls if the terrain is high enough outside or if you can get up to a high enough elevation inside the city.
If you really want horses in the cities, mod it in. We decided to have stables outside and not allow them in. I honestly don't see why this is such a bone of contention, unless it's just complaining for the sake of complaining. If you want a lore explanation, maybe there's a directive from the offices of his Imperial Majesty saying that in order to foster a more civilized Cyrodiil, horses and other beasts are banned from the confines of the cities.