I'd like to find an answer to this question as I can't see how something like this can be happening. I can see three reasons that they would earn that much but any of them if thought better is wrong.
1) They generate all the money they win. I don't deny that they generate lots of money, but news like this one: http://www.euroweeklynews.com/news/axarquia-malaga-east/item/126022-la-liga-clubs-threaten-strike, or this one http://www.forbes.com/sites/mikeozanian/2015/01/22/nba-adding-1-1-billion-of-debt-while-getting-its-best-credit-rating-ever/ make clear that there are going to be a lot of times where their salary is clearly overvalued, and according to only economic terms they should earn maybe something that's a third of their salary.
2) They generate the money on its own. I consider that also mostly false, if they generate that much is because mass media is always talking about them and showing them, if they wished they could take any other thing and with time put it on top being that thing much less expensive for them on the beginning.
3) What the do not earn economically , they earn in entertaining people and making them less aware of their problems, ok, so do a lot of other things, like computer games or books or whatever, and none of them get that much of hype usually, moreover, they are likely to be victims on piracy with a lot more frequency than, let's say, a televised sport event.
Maybe this is applied for other professionals like top actors or singers, but I don't know of such huge debts on their case, and I get the impression that there much more top sport players with huge winnings rather than actors or singers.
I'm not a economist, but I find this question intriguing, hope you can help me to understand why.
Thanks for your time.