Harmonix got the rights to the masters, I assume it's Jason Newsted that you're hearing for the first time there. With the master tapes, they have all the stems, and they're remixing the songs for the game. There would be no reason for them to have a studio guy re-record the bass part and throw it in there (although that's totally cool in Sharon Osbourne's world).
As for why Metallica doesn't do a remix of the album, maybe they're lazy, maybe it's 20 years in the past and they don't really care, or maybe there's not enough in it for them to bother rehashing that. Remixing (not to be confused with remastering) is kind of a big deal anyways, the final mix of an album generally has a lot of input from the band and is what constitutes the "sound" of an album. For better or for worse, it's what they were thinking creatively/artistically/stylistically at the time. Going back and changing the mix is kind of like going back and screwing around with an author's books or an artist's paintings 20 years after the fact, to "make it a little better." I think it's generally a bad idea. See Lucas, George.