The money is flowing into exhibition matches because there are famous names getting involved in those matches.
They are creating a good hype for those matches and hence the tickets are being sold anything.
For example: Logan Paul vs Mayweather match was really hyped and gained a lot of attention.
For old boxers, of course, titles are no longer the main focus for them, they will offer to compete in exhibition boxing fights if the pay is worth it, as in the fight between Mayweather Vs Logan Paul which is quite sensational and strange because in terms of achievements, it is clear that the fight was not worth it, but because Paul Logan has a lot of money and his influence as a YouTuber will certainly be an attraction for the audience to watch the fight, so that Mayweather does not hesitate to accept the challenge and earn quite a lot of money eve though their boxing win record is very much different, namely Mayweather 50 wins while Paul Logan 0 wins.
Funny to say but nothing is needed to compare it's all because of money that's why Mayweather accept that fight, the fans or maybe we can say audiences are the one who push those two known personalities to make an exhibition fight which promoters see the advantage that they will earn huge money from this kind of exhibition fight.
No bearing and Mayweather is assured to pocket millions of dollars here, just a practice or sparring type of fight, but he will be paid by millions of dollars. That's how wise and greed Mayweather was.