Maybe something to do with the quantity you would need? You would need loads for such a big animal and it can be quite poisonous if licked off - the tiny amount for a cat is usually absorbed pretty quickly but a horse would be running in it!. Also possibly different chemicals than aren't effective via bloodstream absorbsion? Just guessing!
Technically there is - at least, there are products like that for sheep and cattle. But I suspect it isn't available for equine use due to licensing regulations, plus the fact that you don't eat dogs and cats...
There is in some ways as cattle re wormed by pour on - however it can blister the skin (learned this when pony ducked as I squeezed the trigger on the drenching gun and it sprayed all over her back. You'd need to much and it would be horribly messy