What is absurd is that the long hours juniors do in investment banking are terribly inefficient.
First because they spend them on tasks that could be automated or outsourced: compiling financials, formatting powerpoints (Microsoft shares a large part of responsability for their poor UI!).
Second because the more tired they become, the less efficient they are at what they do. It is often the case that they would probably get the same done quicker by calling it a day at 9pm and coming back the next day with a fresh mind. But peer pressure prevents that.
Third because even though they spend their time at the office they actually do not work continuously. Them staying late is more often than not because they were assigned something at 5pm with an unrealistic deadline. In this business managers usually disregard the implications of when in the day they give work or feedback. And I suspect it is often because they have been there, done that, as juniors, and have little sympathy for the juniors' time spent at the office. It's a kind of rite of passage.