Yep so many ways to lose in MMA, that's why a 20-0 MMA fighter is incredibly rare, not so much for boxing.
I actually blame this on boxers padding their record and not facing threatening competition until after being 20-0 etc. Not often a boxer fights for world titles like Usyk or Lomachenko unless they have the amateur pedigree they come with. On the flip side, there are many 10-0, 14-0 etc MMA fighters, and that's because they follow the same padding blueprint as boxing, the only difference, the road to the top in MMA is a lot shorter than that of boxing. That's my theory anyway. Many ways to lose doesn't have much to do with it IMO as so many fighters pick and chose favorable match ups until they're in the big leagues, this is even more so evident in boxing big leagues. And the majority of these 'Undefeated' fighters, have a robbery or two on their record because of politics, these undefeated guys are generally popular, get hometown/favoritism with match ups and decisions and are smart enough to avoid match ups that aren't in their favor until they've 'Made it to the big leagues', once they're making bank, the 0 on the record isn't as bad as it was on the way up.
I say this because I have seen coaches protect their own fighters for long term interest and I've seen coaches throw their fighters in the deep end because they believe in them, I've seen the risk pay off, stars born and I've seen the ugly side where some people get pushed too early or aren't strategic with their career and match making (the guys with the, I'll fight anyone, anywhere attitude and accompanied by a coach with an ego to match) I've seen them crumble.
A good coach will take calculated risks but would never chuck their guy in with a fella that is way more experienced. Now for the Coach on that other side, the coach with the more experienced guy, the one from the fancier gym, with all the ties in the sport, they usually have the favoritism, edge in experience against their competition and they're strategically taking these fights that favor their athlete because ultimately it's better guidance, approach and generally a more established gym. Doesn't mean they always win, but those that side and come up through their systems generally have a better chance. At the high level, this translates because, the amount of fighters that have missed out making it to that level or been used as a stepping stone for the 'undefeated guys' almost always comes down to a combination of poor coaching and the clear favoritism politics that begin at the grassroot level and transcend all the way to the grand stage.
