I don’t watch tennis at all.
Although I don’t question your statement about big name tennis players being lefties, that makes no sense. Aren’t they like golfers - solo practioners, one man/woman corporations who don’t eat if they don’t win?
Or are tennis players coddled through life by some kind of tennis syndicate?
I'm not really sure; not the biggest tennis fan. I can't explain why the dynamic exists, but I know that it does. It could have something to do with the volume of non-American players that participate in the sport professionally. If you think about it, there are only a handful of American pros, relative to the field, unlike professional American golf that is dominated by mostly American players.