Comparing boxing and the historical understanding of that structure against MMA is literally apples and oranges. The guys in MMA you're referring to will retire with generally way less than 40 fights. The one's in boxing you've shown have 100+, with many of those fights being 30+ mins.
In Khabib's case, he has more scrubs/regional warmups cited as "professional" than he does fights in a real "professional" league. 13 fights in the pros. That's barely an anecdote really.
The boxers you note typically fought dozens and dozens of guys on the regionals before getting noticed and offered bigger shots. As these men were literally "prize fighters" and making nothing like you see today, many fought regularly just to feed their families.
When viewed objectively, that 91-1 is nowhere near as impressive as it might generally imply.