Stuff from Japan tends to be in better shape, yes, but no one's mentioned one big factor in why American games get so much wear and tear:

The packaging sucks ass.

Seriously. The OP mentioned Saturn cases, which, like their Sega CD and early PS1 brethren, are made of cheap-ass plastic, and break easily. TurboGrafx games came inside those cheesy boxes, with some later games not even getting the jewel case inside. NES boxes, like the game paks inside, have quite a bit of empty space. Super NES and N64 boxes are the same way. And the later-era Genesis games have those shitty cardboard boxes. Even the Game Boy line has a history of oversized packaging with tons of empty spaces that just begs to be crushed.

I'm not saying that US gamers aren't slobs, but the packaging here (until the last couple of generations) just begs for abuse, which simply doesn't happen in Japan.