Limiting selection to within degree holders very well may be irrational, but that's not what we are talking about. We're talking about the ability to fill the jobs that need to be filled. Since, as you claim, they have no need to look beyond those with degrees, then we know fundamentally that they already have every single person they need. That's basic arithmetic.
Again, what does adding another person with a degree bring?
When you claimed that "looking outside the box" was an option. It isn't. A degree from Directional State University might be intellectually equivalent to one from Harvard, but socially it's not even close to being the same thing.
You are arguing from the standpoint of humans as homo economicus, in which companies will hire the cheapest workers regardless of outside factors. That may be mostly true for a sweatshop factory, but it is a long, long way from being true for investment banks.
Limiting selection to within degree holders very well may be irrational, but that's not what we are talking about. We're talking about the ability to fill the jobs that need to be filled. Since, as you claim, they have no need to look beyond those with degrees, then we know fundamentally that they already have every single person they need. That's basic arithmetic.
Again, what does adding another person with a degree bring?