Because at Oxbridge interviews aren't sales pitches, they're assessing your ability to think as A Levels only test your ability to regurgitate a textbook.
In my (admittedly limited) experience the work is no 'harder' than anything I've had, there just tends to be more of it. That's not necessarily a good thing, being able to pump out essays doesn't make you any better at your subject. I think the thing that stimulates more than any other is the tutorial system, students can't get away with not doing any work unlike in the bigger classes of other universities.
Hate to disappoint all the"bright young things" who feel so privileged to be at Oxford, but having graduated a few years ago from there (PPE) I can look back dispassionately and compare myself to people from other good universities. Now we are all mixed up in the big wide world, there is little difference between us. Lots from other universities have the same A level grades and do as well or better than the Oxonians. Someone from University of Hertfordshire got a promotion way ahead of the Oxbridge lot! I have no links with Warwick, but my perception is that their graduates are a very bright bunch and skilled in applying what they know. Some of my Oxford friends have had real success (and I'm fortunate to count myself in that group), others have floundered. Some are incredibly bright-others I wonder how they got in.
As regards intellectual challenge, I think the tutorial system is great for developing the ability to collate information rapidly, present an argument effectively and debate. On the other hand, it means that I have amazing knowledge of the limited areas I did essays on, but my wider understanding of my subjects leaves something to be desired. I wasn't a library-based intellectual though.
I'm glad I went to Oxford but think I would have made a similar success of my life had I gone to the universities which have been mentioned in such disparaging terms.
And a message to the OP - this is the sort of exchange which gets Oxford people a bad name and can make post-Oxford life very difficult (and post Oxford life is hopefully a lot longer than those nine 8 week terms!).
I'm answering this as a somewhat clever but not hardworking sixth former with offers (after interviews) from Cambridge and Imperial:
the interview at Cambridge pushed me about 1000x further than the Imperial one. At Imperial I was set two logic problems, I solved one with ease but with the other one I got extensive hints. With Cambridge it was TWO half-hour intellectually rigorous interviews which made me really think hard.
So on that basis I can make 2 assumptions - I) they do pick la creme de la creme of students, rather than just the best because so many brilliant people got rejected from Oxbridge but not from other top 10 unis cos Oxbridge NEED to see you're amazingly clever not just an A student; and II) if their course is anything like their interview then it is more demanding and thus makes students reach their potential.