Spielberg's newer films are intentionally grainy, and they are beautiful for it. Its the style of DP Janusz Kaminski. His older films weren't meant to look grainy but a lot of the film stocks back then simply were like that--for instance, Aliens is pretty grainy, and it'll never be better unless you start erasing grain artificially but then you are erasing part of the original image.

But you are right about JP--the DVD transfer sucks and theres a bit more grain than there should be. Its dupe grain, not grain on the neg--grain on the neg should be left in but dupe grain can be nasty.