It is not even true that other countries' teams play more together than the USA. The main players in almost every relevant team are playing in the NBA and only attend the call for the World Championship, the continental qualifier and the Olympics. Not even the better guys playing in Europe come back to SOuth America to play the panamerican games or the the South american championship.
Basically, the other teams have players in different leagues, practicing with lesser players and used to different play styles. The americans play all in the same league and face their USA Team fellows at least twice a year, but generally many more times (games + preseason + practice + All Star weekend)