Of course factor in the C17 is effectively an ocean away from the European TOO so needs that extra reach.
I think its two factors:
1. US "strategic" airlifters need to cross an ocean. European ones don't.
2. US force structure includes many heavy units that are expeditionary. Europe's heavy units are all garrison forces and its expeditionary forces are light rapid reaction forces.
The requirements are quite different and thus the airplane that results is quite different.