People posting in the Universities forum are mainly applicants searching for advice, and it's just that intimidating reputation of Oxbridge which attracts more users seeking for help or advice with their application than other unis might.
Oxbridge are amongst the most competitive unis in the world (not just England) and have the reputation of letting only the "best of the best" in (so they say on their website somewhere), so it's natural that people freak out about them like crazy, which is why they join here. (It was the case for me)
That's all, I think, and other than that, I haven't noticed that people on here are obsessed with Oxbridge, or that there are lots of "conceited" Oxbridge students. It all depends on which part of the forums you are lurking about in.