Because your pay is high too! If you can find people to work beneath you for cheaper, why can't they replace you for less money?

Edit: also, they do that with intern/entry-level banking jobs. Internships pay well, but it's nothing like what higher-level bankers make. Same with entry-level analysts.

The senior bankers who actually bring in clients are worth their weight in gold. 0 chance anyone in a position like that is going to stick around to get underpaid while bringing in business.

Because it winnows the wrong ones. They want the extremely tenacious money-driven type, because those are the ones that will help them make billions once they work their way up—and gladly clap on the golden handcuffs as they do.

If you lower the pay, the "best" candidates get grabbed by competing firms. The funnel works because you're not just selecting a subset of people, but because you're selecting a particular group - specifically, those who are willing to give the firm a better share of the organizational rents.

Well it's already $0 in some situations, so... charging people to be interns? You just might have a future in finance.

Not in finance they're not. Finance wants the best interns who can actually perform at least some work, and doesn't want to get busted for breaking employment law in regards to unpaid interns.

GS interns get paid loads