Suspension of disbelief is largely a relative experience. The prequels are so utterly CG - both foreground and background - that the whole "stylized" look sort of reinforces itself and you can get used to it without it looking horrible. Some people can still see that looks fake (I'm certainly one of them) but it's difficult to tell just how much so until you cut back and forth between prequel footage and OT footage. When you do that, the prequels suddenly look like cartoons.