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 despite their 50-0 boxing victory record.