As for 7ft vs 9ft and why Americans suck...
For the players currently playing in the MC, sure they all play plenty of time on the big tables. But as for why we have to drop down quite a bit after our top two players (and take a gift from team Europe to have a reasonable top-3), one has to look at the state of American pool, in general. Why aren't we developing more top players?
American pool has two big strikes against it: 1) most tables are in bars or environments not friendly to kids, and, 2) most tables are 7'. For point one, it means very, very few kids have access to tables during the developmental years that would allow the game to be second nature. Shane was one. But it's very, very hard to be internationally competitive if you pick up the game at 15/16. You have to start at like 6 or 8. And for many, they hardly play regularly until they can go to bars, so start even older.
For point two, you have a bunch of people who played rarely or sporadically during their childhood who are now developing their skills on 7' toy tables, learning to softly plink balls in, but never really having to develop a stroke. So that further stunts player development.
I'm not at all surprised that the US produces so few top players given the two above handicaps. Until and unless there's a more established pipeline where kids can learn and play and have fun from a very young age, and play on bigger tables to help develop all the skills of a good player, then the US will continue to underperform internationally. After Shane declines, the US might be a joke for a long time on the world stage.