Their backs will lack the required hunch. Their hairless tongues will be unsuited for wiping fallout particles from the wheels. This is why we live with the horrid effects of riding our steeds. In fact, these effects are themselves nothing other than the early stages of devolution. One doesn't have to be interested in post-Apocalyptic living to devolve.
Devolution is fun.
Swinging from the trees and chest-slapping is fun. Slithering through the muck is a blast. Grunting and hooting feels great.