What do junior workers do at these banks that require them to work 80 hour weeks?Nothing worth it. I've talked to people who have been through the investment bank meat grinder and who got out. One guy went to Silicon Valley. He said tech firms have their own issues but treat him far better.
The gist is: there's a culture of abusive working practices that appears to have becomes self-perpetuating in the way that hazing rituals at US colleges or the army sometimes do. Much of the work is horrendously inefficient, for example, writing long reports for managers that are then never read, or which are read and then ignored, or in which only a single figure is actually needed. Other problems involve managers who have no idea what they want so constantly change their requirements, causing work to be scrapped and redone, but with arbitrary and ludicrous deadlines attached.
Additionally managers think nothing of setting someone a new task to write a large report at 9pm on Friday night, then demanding it be delivered to their private home at 6am on Saturday morning.
Because the workers are young and ambitious, they tolerate this even though they can see that a lot of it is work for work's own sake. And presumably the only ones who make it are the ones who have become inured to the notion that arbitrary work is some measure of self-worth, and thus try to select "the best" by pushing people to the limits and seeing who breaks.
This culture is entirely absent in tech, where if someone is asked to do a task there's a general expectation that the task actually matters and has been thought about - at least a little bit.