IBs and consulting firms tend to hire from Ivies and other elites because the people who do the hiring are mostly from these schools. They’re personally biased toward graduates of their own alma mater, or schools similar in their mind to their alma mater. The same applies to most large SV firms, except their current employees already come from a much wider range of schools. Smaller SV firms may still have their own preferences/biases for the same reason.
As to the usually high salaries for freshman summer interns at a few elite colleges, my guess is that they are in a highly desirable technology area, even if they’re hired by banks and hedge funds. Banks are highly concerned that they’ll be dis-intermediated in a not-too-distant future by technologies (a number of SV firms are trying to do just that), so they’re bulking up in those technologies, trying to stay ahead of the curve. The same technologies have also become increasingly important to many hedge funds. Their competitions for these type of talents are driving up the cost.