Because it's cool to hate on it. :grabs umbrella for downpour of down votes:

Because from a literature standpoint, especially a vampire novel standpoint, it's worth less than poop on a shoe.

But that's the thing. It's not "literature." It's a novel for young adults meant to be a fun, casual read. Sure, some people take it too far. Whatever. That's what happens in fandoms. I'm not defending the OP's girlfriend's decision, and I'm not even a Twilight fan, but it really shouldn't be as big of a deal as it is. I read the books, and they were fine. It was an alright series that kept me busy for a few days. Nothing special. I think I rambled a lot, but my point is that light reads don't need to have a literary basis. They don't need to be life changing. Their purpose is to entertain.