You have to remember that one of the main reasons we need to brush our teeth is because we are eating foods we were not meant to eat. Humans are not designed to eat potato chips, soda, and cookies, so we get cavities pretty easily. Horses, however, are eating foods that are healthy for them in most stables (hay or grass) and therefore don't have the teeth issues we have. I suppose if you fed a horse enough sugary feed you could get issues, but you shouldn't feed a lot of sugary feed in the first place. Horses need their teeth checked once a year for floating, so you can ask the horse dentist how the horse's teeth are holding up. I don't think horses need their teeth brushed in normal circumstances, if at all.
"The trouble with the world is that the stupid are certain and the intelligent are full of doubt"