Just to say I’m fascinated by the OPs original question.
My wife rides horses (and bikes) and has a lesson every 1-2 weeks, pays £18-30 depending what it is, who’s coaching, how many other riders.
I’d happily pay £20 for an hour’s ride with 2-3 other bikers of a similar level and a decent coach as long as it was local – 15 min drive max.
Maybe reason it doesn’t exist is that many ‘recreational’ horsists do aim to compete, even at a low level, and are judged on technique. Lots of nursery/novice classes around in SJ and dressage. It’s kinda the only way up from riding in your own yard’s arena.
By contrast, a recreational MTBer may aspire not to race but to go for a weekend in the Lakes or in Torridon, or to ride a big drop in the local woods. Even if you do want to race and choose something easy to enter, if it’s Mountain Mayhem you really don’t need good technical skills to get round. Nobody judging technique and giving you a scoresheet either!