Horse breeding would be relatively easy to implement.
You've also got to remember that people in dev teams don't have all the skills, a few might, but most are specialized with perhaps basic knowledge of another field or two . You can't get all the team to do that thing you want (and it'd be counter productive in some spaces, like having three people on the same keyboard). Working on seemingly minor improvements is something that can be done whilst a few people tackle the big things. I for one want horses to be a resource, so I'm happy. Other than the way reinforcements could work in battles (seriously, make it more natural and intuitive, and give me a chance to skirmish against larger armies or play my retreats. The difference would be profound) and how I don't like how oversized buildings are even within the context of a third person action game , every big sweeping change I've wanted or didn't know that I wanted seems to be in the game.
Some of those horses look great, some of those horses look bad. The game'll probably look more consistent at release.
If the Devs could make carts,chariots, , cars,planes, flying horses, birds, giant bats, submarines, helicopters, dragons and space ships possible then that'd be really nice of them...
Also, you gotta remember that people'll do stuff they're motivated to do better.
Someone who really loves horses would be super keen in adding that stuff, they'd probably do it for free!
In contrast, if you had them model medieval european wear when they're not interested in the subject, you're not going to get good results.