A lot of it is ego-driven. Senior people think that because they went through it, all junior people have to go through it too. Companies feel that if they're shelling out huge money in fees that banks should be on-call 24/7. If you're responsible for a deal either from the company or adviser side, , you'd rather have it fixed before anyone notices or at least as quickly as possible. Thus the benefit of junior staff working crazy hours.
What it really sounds like is that a lot of shit work is being done all over the place. If everyone needs to be on-call 24/7 and most of their time is spent applying fixes to problems that are continually happening, something is probably broken in your organization.
