Well I do MMA, and from other discussions there are a few others here who 'do mma'. So far more than 1% of 1% of 1% of LetsRun - a running forum - 'do mma'. Personally, I know virtually zero males who haven't done some kind of combat sport training at some point in their lives.
It all depends on what you mean by 'do mma'. For a start, you're making the mistake in assuming that because (according to you) there are relatively few people 'doing mma' then the best in mma are the best of a small talent pool. It can still be the case that the best in the world are doing MMA, and Khabib is the best of them (and hence the world). Just because there might be a billion hobby joggers in the world compared to 100,000 or whatever 'doing MMA' in MMA gyms, doesn't mean that Jakob is necessarily a better athlete or from a wider talent pool than Khabib. The people doing MMA are likely to be the ones talented for MMA, and that includes the desire and capacity to give and receive physical trauma. You could take the entire field of London and Boston marathon competitors combined, and not one of them likely has a prayer of ever beating Khabib even if they had been trained in MMA since they were teens.
The top MMA competitions (namely the UFC) attract some of the best athletes from multiple sports - such as Olympian wrestlers, boxers, kickboxers etc etc.
A sport dominated by a tiny region in East Africa, and that we now know largely due to rampant doping there, and that pays the best maybe $100K a year, compared to $10 million a year for the best in MMA, isn't necessarily a bigger talent pool just because every kid 'runs' at some point.