I think part of this comes from the way MTBing has developed in this country. The club culture that exists in cycling is something most clearly identified with road riding.
MTBing went through an extended period of "roadie bashing" (hence the nonsense threads on here about miserable roadies), and patr of that I think has contributed to the idea that formal organisation of TMBing is against its core principles.
Now there has been a resurgence in road riding (whereas MTBing has been growing and growing in any event), there are more people who want to ride with road clubs and get into contributing to the huge number of races and events put on by road clubs that way.
For so long as MTBers eschew the idea of organising themselves under umbrella organisations, there will continue to be an obviousmajority who benefit from the hard work of a committed minority. That said, that's true of almost every activity. I just think it's more obvious with MTBers, who tend to organise themselves along the lines of riding with friends, rather than riding in clubs.
For me, I ride MTB very little these days, but have enjoyed the benefits of others' hard work at the couple of races I've done this year (Mayhem and HTN x 2). I contribute to my (road) club's events, and help out at those, so am doing something for cycling.
It's good that people do trail clearing and maintenance, as that's proper thankless stuff – non-riding effort for others instead of riding ne's own bike..!