UFC athletes simply are not up to par athleticism or skill-wise compared to those of other combat sports, let alone major league sports. We saw how Israel Adesanya and Alex Pereira came into the UFC and blew through numerous champions and championship contenders just with some rudimentary TDD knowledge. We saw how highly ranked UFC fighters Ben Askren, Tyron Woodley, and Anderson Silva all got dropped/folded by a Youtube influencer. And now MMA's final hope Ngannou got knocked out cold.
And before you say that so-or-so other combat sport athlete would not last a minute against UFC fighters in MMA, never forget that Ray Mercer knocked a former UFC champion Tim Sylvia out cold in his prime. Nobody in MMA has ever achieved the converse - Ngannou was the closest but we just saw how that went.
The truth is that the UFC is barely a step up from BKFC - it's just mandated brawling under the guise of combining martial art styles. What do you expect when C-level boxers and kickboxers (which all UFC fighters are) go up against A-level boxers and kickboxers?