The thing is there always used to be one of those. There was a Borg, or a Lendl, or a Sampras, or a Federer or whatever. The next generation seem to be 90% mugs.
Again, even in the poor basketball or football or baseball/whatever clubs you get decent money. Not only that - you get head coach, assistant coach, goalie/big men/etc coach, physios, doctors; your accommodation and traveling is arranged, pretty much you just need to play.
From the little I picked up reading the forums, many players - especially younger ones/who didn't break through can't afford a coach or a physio, travel alone, can easily lose a match just because they were too tired or couldn't find a decent place to hit or a decent hitting partner. AJ mentioned in other topic that Challengers money didn't go up in the last 20 years or so.
How many top sports talents would even consider picking up tennis these days, when they could be so much more successful (and rich) in many other sports.