Those guys got into boxing before MMA was a factor. Tyson himself said he would have competed in MMA today. Its pretty obvious that MMA is taking a decent share of the combat sports talent pool nowadays. When someone first gets into a combat sport (as a participant) they're usually too young to be thinking about money long term, its more about what they've been exposed to, what they think is cool, what athlete they want to be like etc. Boxing used to be the only game in town in all those respects.
You haven't refuted anything I've said. If you have monster talent, you take your talent and make the most money from a sport that gives you the most opportunity. Boxing's top talent makes far more money from purses, a cut of the PPV percentage, and most importantly endorsements. What kind of archaic shyt prohibits you from using your image, name, and likeness from making money? Hell, even top amateur athletes make far more money than UFC fighters and champions, considering they can now use their image and likeness to endorse products through the NIL law. You said some guys got into boxing before MMA became big? MMA became big and made a name of itself during the mid 2000s, around I say 2004. Pacquiao and Mayweather were in their mid 20s then, why didn't they jump into UFC or any MMA organization and make money? Because the money is so low and not worth the risk. Top elite boxers like Pacquiao also has endorsements, something that UFC fighters cannot do and are prohibited.
The biggest names in boxing right now are Fury, Wilder, Canelo, Joshua, Inoue. Those guys I mentioned were born late 80s to early 90s, they were all little kids or teens when MMA became big. How come they didn't jump into MMA? Fury, WIlder, and Canelo makes 10+ million per fight, Inoue doesn't make that money since he fights in a lighter weight division that doesn't get the attention, but he's able to use his name, image, and likeness to endorse products in Japan where he's massively popular.
If you have massive talent in using your fists for combat, you take that talent and get into boxing where it's far more lucrative not MMA. Even a youtuber like Jake Paul got into boxing and instead of MMA. Washed up UFC stars are begging for a fight with him to make some decent cash, because when they were in the UFC they weren't making shyt.