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.

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.