Because his chances are not the same anymore just like his power, speed and defense maneuvers. He cannot hide that fact that he is already old even if he's still fight in exhibitions, he will have the ring rust effect if he will choose to deal with the likes of Pacquiao nowadays, a May-Pac fight is profitable but knowing Mayweather, he doesn't want to get embarrassed from being defeated.
He doesn't really taste defeated ever since so what do you think he would feel if he gets defeated by Manny Pacquaio, or any boxer there is or should I say a Youtuber or an MMA fighter defeating Mayweather that level of embarrassment he would feel, because he hasn't experienced it at all, so experiencing it now could be a different effect for him, I'd say I'd really like to see that he gets embarrassed by a non-boxer inside the ring and the sports that he loves so much, I really want to see his reaction, and what would he feel,
That is true and I admire him for that because up to this day, he's still clever enough to know what should he do that is right and what is not, considering the hundreds of millions they can acquire if they decide to have a second dance in the ring. Bottomline is he cannot risk the chances of getting defeated af this point knowing how competitive Pacman is.