It's definitely about the money and the risk-reward ratio. No one is burning thousands for tennis lessons for their son to barely break even as a 100+ player. Team sports pay much more in the US for a much lower "ranked" player.
I'm sure with the population of the US it's possible for unnoticed talent to be spotted sponsored and nurtured but as others have said teaching methods and the lack of clay courts might have rendered them technically backwards in today's game.
same applies to Europe.
Football, top 100 player, is just as much as roughly the main squad of top 10 teams.
But even outside the top 10 teams, folks get Mio / year in the biggest leagues.
England, Germany, Spain, Italy, France + some of the reach teams outside the top5, so you have 5 (top leagues) * 20 (teams) * 25 (players per roaster) = 2500 players that earn at least decent money, if not being super wealthy.
And I think I am not much wrong if I think that actually, to compete (money-wise) with a tennis pro on the brink of top 100, a football player doesn't need to be in the 2500 players mentioned above, it is enough to be in the main squad in one of the teams of on of to 40 countries. Which in turn is at least 10'000 players, if not more.
Basketball.
It is as well a big sport in Europe.
Ice Hockey.
It is as well a big sport in Europe.
So, I am not sure that best talent in US goes to team sports while in Europe best talent goes to tennis.
Those who are looking for money, could as well go for team sports in Europe.
To my memory, both Federer and Nadal made a choice between tennis and football at some point of their junior preparation.
And I recall similar stories from more top pros in tennis.
There must be something beyond money.