Okay, call me a scavenger, a lowlife, even a poacher. I like to think of myself as a recycler. With so few unclaimed horses in the waste, I've come to prowl other places in search of breedstock, and have found a lot of derelict homes with bunches of horses that are just going to get deleted when the admins get around to it. Why not make these horses go wild, so those of us recyclers can make use of them? Their plots are void anyways, why can't I get use out of it?
I think the answer is the same when it comes to the other materials on the residence: because those belong to another player. And until his home has actually been removed there is always a (slight) possibility that the player comes back to reclaim all his stuff. As to re-using. Let's say this would come to pass, then I think it would put a lot of (unneeded) stress on the staff. How would they make sure that the materials get divided equally and fairly? A bit skeptic / harsh here: what gives you more rights to those free horses than me? I think all this would do is introduce a lot of drama, more work for the staff and, yah, in the end unneeded. The wastelands have plenty of horses. Especially next week after the reset!
Also, if you really need some great horses. It's not that expensive to find a decent horse at a shop. There are some very good shops here and there with all variety and many speeds.
... As someone trying to make money in the horse business, buying the horses is exactly what I'm trying not to do.