Spotting talent in teams sports is much less of a risk than in individual sports. In team sports the (mental) maturation aspect is partially voided or, in some cases, almost irrelevant so when you see a young talent you can have a better idea of how they'll progress as they age/grow, making the gamble a better bet (plus team sports have more of an element of 'gamble' money to sponsor promising juniors). In individual sports that is not the case - the mental acuity required to play a sport like tennis at a top level when young is a hundreds times rarer than being a great free-thrower, goal-scorer etc.
There is much less certainty that a top junior will turn into a top adult tennis player, especially as mental maturity (in a sport-specific sense) matters so much more in tennis than it does for a team sport.
An example of how difference sports force people to mature in order to be top competitors can be seen in how many complete morons there are in high level football (soccer) and American football. Yet they are few and few between in tennis or golf players*. You just couldn't survive tennis or golf like you can many team sports as the group/team collective mind/confidence aspect almost doesn't exist.**
(*I don't think it's a rich/poor sport thing either in case someone was about to suggest that aspect...)
(**Ever wonder why so many historic anomaly results in tennis have come during Davis Cup? I think a big part of it is because some players become more confident and perform better when they have team support around them - especially on-court support)