Its often a combination of two things. Ignorance and Denial. "He's old" "He's a hard keeper." "I'm doing everything I can." "My other horses are healthy!"
IMO, with all of the information (now so easily available) about feeding, equine medicine and management, there is no reason for a horse to get like this other than laziness. I don't think most people intentionally neglect a horse, but they sure don't put much effort into correcting the problems either.
There are some who also just don't think about what it really means to own a horse. They are the "wouldn't it be fun to do" mentality. But, they don't think of all that is involved in owning one. They don't think about the fact that a horse is not a dog. You can't put a horse into the car and take it to the vet. There are so many things they don't think about. They just get them and let the chips fall where they may.