So, it sounds like you're saying they should first hire a team of team of scientists to actually make the rage virus a reality, then make the movie, because making a movie about something that doesn't conform to reality is sloppy film-making.
Asserting that works of fiction can't take any liberties with science is pretty obtuse. Since your standards of credibility are impractical, and most folks don't have this mental inhibition, I posit that it would be sloppy film making for them to waste time catering to those standards.
Furthermore, your disposition that we actually know for an absolute fact all of this stuff that the body can and can't do belies the fact that there's still quite a bit that's unknown about human biology, particulary from a neurological point-of-view. We don't have an owner's manual after all. What we do know is largely based on observing the body as it's subjected to different conditions, and if it always seems to react the same way time after time, then it's regarded it as an absolute, which is convenient but sloppy. All it takes to break that absolute and get scientists all excited is that one new condition that triggers a different response.
But really, all of this is moot if you're giving George Romero's approach to zombies a free pass. A dead organism managing to get up and walk around and attack people without requiring any of the biological processes that allows it do so is pretty far-fetched too, so it sounds like you'll suspend belief if a movie's utterly preposterous, but not if it's merely a little preposterous.
