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.
