A lot of reasons. Chief among them are horses smell like ass, are jittery and unpredictable, are ugly, and have the creepiest eyes of all farm animals.
In general, I only hate two types of animals: those that are easily spooked, and those that are completely selfish -- if I'm going to be around an animal, I'd like it to acknowledge my superiority and be unconditionally happy with my presence. Horses definitely qualify for the first group, and sometimes for the second, although they're not really pets, so we'll cut them some slack in the latter category.
All of that, and the fact that a horse got spooked while I was bareback riding it once, causing it to race uncontrollably at break neck speed, disregarding my commands (keep in mind I was a newbie), hurtling over fences, bruising my balls and plowing (pun intended) through my uncle's newly sprouted maize crop (yup, maize can haunt one many ways), destroying a large section of his field and earning me days of severe reprimand.
Yup. I f-ing hate horses. To ice the cake, they're boring as *&^%. Horse racing is especially boring. I don't give a *&^% about thorough breeds and the rich people that make money off of them.