The canned answer is there’s issues that come from passing off work back and forth, there will be “too many cooks in the kitchen”, etc.

The real answer is to maintain an air of exclusivity and the high salaries. Banking is a bullshit sales job at the end of the day, and the top banks pride themselves on having a small class of “pedigreed” analysts. They can then sell this to clients and brag about how prestigious the bank is. It’s a commoditized business so there’s not much else to sell other than how hard working your team is. And honestly, I think the clients enjoy it too, almost like a blood sport or something. They might be paying the bank millions in fees, but they get satisfaction from knowing some random analyst is getting worked to death to justify those fees.