They are aware of this, but focus on that small portion of the population anyway because the people that can go work at an investment bank provide a ridiculously disproportionate bang for their buck.
Tangentially, I think you're underestimating what a typical Oxford grad can do. Most of them absolutely can do one of those things, especially if they decide to before they graduate.
I went to Cambridge. No way could I have afforded an unpaid internship at a thinktank after I graduated.
Possibly I could do it now, but even if I obtained that golden policy job - how would I survive in London? Housing here is more expensive than even San Francisco.
My degree has opened many opportunities - sure. But I can't pay my rent with it.
Tangentially, I think you're underestimating what a typical Oxford grad can do. Most of them absolutely can do one of those things, especially if they decide to before they graduate.