The thing is, most of the people who get into these top schools don't really *need* the MBA in the first place, especially those who are already in business (finance, consulting, retail, marketing, or any other corporate job). And especially at places like HBS, Wharton or Stanford where a disproportionate number of the folks already have blue chip backgrounds (Ivy undergrad and/or top tier employers), the additional "brand" of an MBA on their resume is incremental more than anything else (and it's an awfully expensive thing to be paying $150K + $200-300K+ in forgone salary for 2 years just for an incremental benefit in "brand"). And given their resumes pre-MBA, they have access and opportunity to get the same post-MBA jobs without even having to go.
So why bother going then? Many of these folks who apply do so not because they need it, but because it's like taking a sabbatical from work for 2 years while they're still relatively young. That can be invaluable - just to decompress and be "you" again without any association with an employer who takes up your life. As such, some may go back to their pre-MBA careers, while others will do something completely different. For someone who takes a purely transactional approach to their careers (i.e. "I will do XYZ in order to ABC") it may seem alien or even stupid to take an expensive vacation like this, but these people, many of whom have worked with or seen up close those people in executive positions (i.e. consultants, bankers, PE folks working with Board members, CEOs, successful entrepreneurs, wealthy families, etc.) can see with their very own eyes what goes on "behind the curtain". A lot of miserable, stressed out, neurotically insecure CEOs, and "titans of industry" that you work with - and that doesn't go away with more money or external success.
Do these people want the money and power like so many other MBAs? Absolutely. But there's also some perspective that comes with actual experience seeing those with money and power -- and that the more important dimension is doing something MEANINGFUL or that has some personal significance. That's why the cliche "do what you enjoy" or "find work that has personal meaning to you beyond money/ego/power" is so real because I can tell you many of these people have seen with their very own eyes what it's like when you don't.
That's a big part why some Harvard/Goldman guy will go back to school - of course on the surface they will say "networking" and all the conventional script that everyone will say, but underneath it all it's also for personal reasons - giving them 2 years of time to really answer the question: "is this what I really want to be doing after seeing what I've seen, or should I try something else or at least explore something else?". Again, through this 2 year process some will go back to the well, and others will find other pastures to graze.
If you want to stay in a business career, it's an expensive but safer bet to "hide from the real world and meditate for 2 years" in an MBA program compared to say, joining a less expensive Buddhist monastery (but again, some people will do that instead -- there are corporate refugees in all sorts of place you'd least expect - on a film set, in a church ministry, teaching at a local college, and doing something away from the grind).
Of course this doesn't account for everybody at these top schools, but a good chunk if not a majority. The engineers, schoolteachers and military officers may get the most benefit from an MBA program in terms of the education itself (i.e. they actually do need it to transition into a business career), but these people collectively don't make up the majority. The majority already have a business career pre-MBA.
Now you may ask why would adcoms admit a bunch of people who don't need the credential - it's sort of like the banks being most willing to lend money to those who don't need the money (credit crisis aside).