Too many people have dogs vs horses. And the size of the latter's poop means you can tend to see it whereas doggy do lurks on the wet pavement/in the park grass/amongst the pebbles on the beach and unless you're specifically looking for it...
Plus dog mess is frankly revolting, whereas horse muck is much more like compost and less, what's the word, squelchy? Horse muck doesn't seem to have the inate ability of dog cack to find its way into every groove, stitch and dimple on a shoe. I've had to throw one pair of shoes out after dog cack got into (above) the outsole. Could never get the smell out of those.