i mentioned roddick (retired and was one of the few successes of his generation anyway).
the bryans are doing a fabulous job, but dubs is different. we're talking singles men.
with all the college players, juniors, sponsorship, access and money in tennis in the usa i can't imagine why you don't have more top players.
Disclaimer: I'm NOT a coach...
Real tennis: https://www.youtube.com/watch?v=yDqnkLJ9BtM