Yeah I have always found this to be extremely weird; the two sports just don't go well together. To elaborate:
Floyd Mayweather Jr. is not known as a knock-out artist, but he has both excellent conditioning and pretty much the best defensive technique of any fighter probably ever. Unless you catch him unaware or get very lucky, you can't hit him. Lately, I've seen two guys do "ok" against him, Miguel Cotto, who just charged and tried to brawl him the whole fight, and Manny Pacquiao, who only had a shot because he's fast and his attack comes from a variety of angles. In the first case, Cotto lost because he took way too much punishment for keeping the fight so close-quarters, and Manny just straight up failed to catch Floyd.
McGregor is a hell of a cage fighter, one of the best right now, but in that scenario you have a lot more at your disposal that boxing does not allow. That, and boxers are trained to control distance and pace, because they know their matches last a lot longer and they don't want to get floored by some unexpected haymaker or gas out too early unless they're going for a quick KO. In a boxing match, for McGregor to win, he'd have to get Mayweather down VERY quickly, and Floyd would never let that happen. If they fought an MMA fight, expect Floyd to do better than you think, because he'd probably control the pace and keep McGregor off of him (if you watched Rhonda Rousey get beat by Holly Holm, Holm used a boxing-like strategy to keep Rousey at a pretty significant distance and just beat her hard every time she tried to get in).
Lastly, Mayweather isn't fighting Terrence Crawford or Canelo Alvarez or Genaddy Golvkin or anyone like that because he has more money than he knows what to do with (dude has a Cadillac golf cart and his own plane(s)), and he doesn't want to have a loss on his record. He has no incentive to risk a violent brawl or be remembered for a dumb publicity stunt like this. I don't personally like Mayweather, I think he's a punk, but he's a very canny businessman and he's obsessed with his image.