Worth noting is that these firms go after mediocre Ivy graduates. The 3.8 CS majors are overqualified (except for quant positions). It's not about intelligence. It's only somewhat about social status. Mostly, it's about the people they're trying to hire: status-hungry kids with an unconditional work ethic. These people make bad hires in technology (unimaginative, egotistical) but are great junior I-bankers because they'll do whatever is put in front of them, even if it's the most awful work in ridiculous volume, for the sake of prestige.
What is the profile of the 25th-percentile (as opposed to 50th or 75th) Ivy Leaguer? Someone who's not especially smart, but loaded up on trophy extracurriculars and averaged a 3:30 bedtime during high school on account of perfectionism and overcommitment. For investment banking, that's the ideal hire for a junior position.