Its not about productivity. the people doing the work are junior and replaced every 3-5 years. they are disposable, in that sense. Since the repsonsibility gets quite steep quite quickly, its more efficient to haze the junior staff, to push them to the breaking point and see how they do, before promoting them. Many people enter banking who are not suitable to it, because of greed or social pressure. It would be a lot worse if those people were not weeded out. it might sometimes seem impossible that banks could be more poorly run, but there are many who would be far worse to put in these positions.
Except that "hazing" arguably selects for those driven by [extreme] greed and social pressure, since anyone smart enough to get into a junior banking position has plenty of more palatable alternatives if they rate "quality of life" higher than "net worth age 30". In theory, if not in practice, ability to rationally appraise alternatives ought to be more important to people making billion pound decisions than energy and sheer bloodymindedness. The only way "hazing" works as a selection effect is if junior analysts' actual work tasks are generally so easy that it won't mentally tax even the below-average ones without sleep deprivation. It's harder to fake competence when you're tired. It's also harder to fake competence if standards are set higher...
The more general excuse for overworking bankers is to limit the number of people accountable for a particular project, meaning no sharing of workload even when it would be efficient to do so.
Interns don't need to be efficient (they're cheap and disposable), and they don't do very intellectually taxing work, in general. Think making powerpoints, not designing investment strategies or coding. There's little incentive for the more senior bankers to treat their interns properly.
That actually makes a lot of sense then. They have an overabundance of interns who are willing to work pointless exhausting hours to prove their mettle allowing them to make bank on man hours.
The question become then, why do clients allow them to get away with it? Do they satisfy their clients to such an extent that they don't care about bloated costs or do the clients just not know?
And for a client, the deadline is more important than the money sometimes. I mean, when you see what these firms bill, it's obviously money is no object for the customer... because damn.
If the job market keeps worsening and huge unemployment/underemployment/unpaid work becomes The New Normal, it will become time to have a hard conversation about hard caps on working hours as a means to improve quality of life for the workers and provide more jobs to the non-workers. You want to put in 100 hours? You'd better be co-owner.