I think you have to look at it's history as well. I mean for longer than any other establishments, oxbridge have been educating and forging contacts with the very top of pretty much every field, worldwide. That's one of the reasons why employment possibilities are so strong from the unis, because employers the world over have heard of Oxford and Cambridge and know the value of a degree from there. Comparatively, unis like Warwick get pretty close in terms of research excellence and quality of graduates in a lot of places, but it doesn't have anything like the international reputation.
This, coupled with the strict entry criteria, help employers. They know that to get into Oxbridge and graduate well you have to be pretty damn bright. 'if they're good enough for oxbridge, they're good enough for me' kind of thing. While you get that to an extent at places like Warwick, LSE, Imperial, UCL etc, I think for the sheer bredth of excellence oxbridge are still unmatchable.
Obviously as it has already been said, their reputation also helps them to keep the top spot by attracting the best researchers, the brightest students and the most money.