Well, one of three reasons as I see it:
1) the testers were told to ignore certain bugs ( can't believe for a second they missed them)
2) the devs ignored what the testers discovered.
3) the publishing wing told the dev's to ignore the bugs and get it as ready as possible to stick to a gimmicky release date, and then spend the next months finishing the game after release.
Of course, the apologists will tell you it is a large and complicated game and a few bugs are to be expected, there's nothing wrong with their copy of the game and it only effects a small amount of people.