To re-heat's point, I work as a banker for a US bulge bracket and assist with the recruitment of new entrants - we only actually look at 5 UK universities, and no further.
If we can take advantage of a sufficiently large group of people already pre-selected to a known (theoretically high) standard, and educated to a known (and theoretically high) standard, why need we go to the expense (time/$) of looking elsewhere? Clearly this can mean that a smaller pool of talent is left unchecked, but the reward of searching these individuals out is simply not worth the investment - a similar approach is adopted by the airlines, and it makes financial and operational sense for any business.
I can appreciate the sense of injustice you may feel. However, no company owes any one individual a job, nor should incur additional costs to cater to a particular method of positioning for that job.