Seeker
Have you ever looked at reviews before watching a movie, booking a restaurant or holiday? Maybe you don't, but lots of people do. I don't have all the time in the world to watch every single film in the world, the stomach capacity to eat at every single restaurant there is or even to spend my precious few days of holidays at a crappy resort!
Well, what do you think are employers doing?!? They do not have the time or manpower to interview the 5,000 or so people that interview for 50-odd places (some more, some less) a year. Having gone to Oxbridge (and a list of other unis) is like a 5 star rating. Everyone knows how difficult it is to get into those universities. In a way, they're already pre-filtered... the cream, let's say, of graduates.
In sport, you might win a race here and there, but everyone would be in awe if you got a gold medal at the Olympics beating Usain Bolt. Nobody would even THINK about suggesting that beating Joe-next-door at a random race would be the same as beating Usain Bolt at the Olympics! Now, if I were to bet as to who would win a race... would I choose the one who beat Joe-next-door or the one who beat Usain Bolt?
The whole Oxbridge thing that you so despise is just the academic version of that. If you are attending such good unis, you are essentially pegging yourself against the best. This impresses employers. Not too different from the Usain Bolt example above.
Yes, it is unfair - but wake up call: the world... is... not... fair. People are not born equal, no matter how much you want them to be so. Our genes alone make us different.