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.
That fight between Logan Paul and Mayweather is literally a straight pot of honey especially for the latter because Logan Paul is a perfect opponent in an exhibition fight, not because he has some talents in boxing and can fairly challenge Mayweather but because Logan Paul has a vast loyal fan base that will really support him whatever he does.
And for Mayweather, knowing that fact alone is already enough for him because he is already assured that their fight will bring them big revenue, and yes it happened.
Although everyone knew the outcome and treated the match as a purely entertainment event it's still funny to know that the match ended in a draw.
This makes the point stronger that exhibition match are mostly scripted otherwise Mayweather would have been declared a winner already.