I don't agree that they should be, but I think it's a sad reality that for *some* firms...they definitely are

. There are definitely factors that are a much better indicator of someone's ability to become a lawyer than the university they attended.
For some employers though, I imagine they just see Oxbridge or RG as being indicative of continuously good academic performance. Although when I started university (RG), I was told the standard of education between different institutions can vary so that might hold some weight too. Could also be absolute BS.
It's just so unfortunate that a system exists where some candidates are judged on things which may not necessarily reflect their individual ability.