Valid points, my dear Dubby. However, horses start and stop when you want them to, and they also go where you point them. Unlike dirt bikes, however, if you happen to point them at a tree or toward the edge of a cliff that you can't see, horses will stop before hitting said tree or careening over said cliff to ask "hey, are you REALLY sure you want to do this?" Horses don't need headlights to see in the dark, and they see much better than we do.
If you are lost, do you take your hands off the handlebars and let the dirt bike find it's way home? I think not.
Cleaning up after a horse is not that bad...besides the horse residue is GREAT for your garden, unlike dirt bike effluent.
Horses eat grass...no cost. Even if supplemented, a bale of hay costs $3.50 and is enough for one horse for almost a week. Let's see....gas is how much per gallon? Housing....well, my horses use a run-in shed, they don't need a fancy garage, and frequently sheltering under a tree suits them just fine. Their paint doesn't chip, and their "tires" don't need to be replaced. Average usable years for a horse is 25 to 30. Average usable years for a dirt bike? Well....it isn't anywhere close.
My gelding is unlikely to produce offspring....so no messy birthing process. Even if you did breed a mare, however, they generally take care of it by themselves, and then you have a replacement for when the original wears out. Do dirt bikes provide their own free replacements?
Easier to transport? Not really. You take a dirt bike to where you want to ride it on a trailer. You take a horse to where you want to ride it on a trailer. No difference. Or, you can hop on your horse in your backyard and ride locally....just as you could with your dirt bike.
Highway speeds? Okay, horses don't do highway speeds, but the whole purpose of dirt bikes and/or horses is to get OFF the highway, so that point is moot.
(hehehehe) Concede yet?