I think it's supposed to be the Hellmouth that draws them there. But as we see on the Angel show vampires are all over the place. Vampires are stupid yes in the sense that they greatly underestimate Buffy. Not just any slayer, cause slayers died all the time before Buffy, and as we see with Spike, vampires do kill slayers, but Buffy is not just any slayer. So many of them are probably thinking like Spike in S2, they can just take out the slayer and run the town. But yes I think most vampires especially the ones we see after S3, are just stupid bloodsuckers who don't think too much about a plan or encountering the slayer. I'm sure they just think they can get away with it. Also there are a lot of people that get turned into vampires already in Sunnydale so presumably they don't know about Buffy. There is definitely too many generic vampires that we see vs the ones that actually have a personality and a plan. So it's hard to tell what vampires are really like as a whole. Are they more like Spike and Angelus or Harmony's minions? They really subvert the demon aspect of vampires as the show goes on, which is a problem for me. I don't know if I answer your question or not, that's just my rant about vampires.