Earlier quoted context omitted.
I have several friends with top tier MBAs. They all got them in an effort to jump from the sell-side to the buy side, and most saw salary increases from 300-400k to 600k-1M+. The ROI on that specific career move is pretty clear and is really the only path I would personally justify for an MBA. They all say the course load and work is a joke. It's mostly an accreditation to the buy side hiring process and networking.
I don't know about your friends but the average salary out of an elite b-school is something like 160k in the best of cases [1], not the ranges you mention... Either they're total outliers, or you're talking about something like an exec MBA, or something else altogether. [1] https://www.usnews.com/education/best-graduate-schools/top-b...
It's by no means necessary, you'll find people at those firms without MBAs - a close friend of mine made the jump directly from banking and is well above the salary range I mentioned, but a top tier MBA helps.
It's also worth noting that a majority of compensation from these positions is from the carry on the person's fund (or a blend of all funds, like Blackstone does) and is based on fund performance. This isn't their base or otherwise guaranteed salary.