I think what makes many Harvard grads behave as @JHS describes is mostly a combination of the Harvard experience and being the sort of person who gets into and attends Harvard (and places like it, to be clear).
Having spent a lot of time at Harvard and been associated with it for decades, I can attest that it can be a fairly competitive social environment, I believe more so than comparable schools. Typically, to join a club or other organization, students have to go through a competitive process known as “comping” (see here: http://www.thecrimson.com/article/2017/1/27/vassallo-quit-the-comp/), or, in the case of final clubs, “punch”, terms unique to Harvard.
The administration historically hasn’t been much interested in bending over backwards to ensure all undergraduates are having fun, either; there’s an old saying that “Mother Harvard doesn’t coddle her young” (I think the current start-stop process of trying to abolish exclusive social organizations is a recognition of this and an attempt to address it).
Most schools at this level have undercurrents of stress and competition; for the above reasons, among others, I think these are particularly strong at Harvard.
I believe what @JHS describes is also an artifact of the types of kids who apply and are admitted to Harvard; all their lives, they’ve been winning competitions. Getting into Harvard was a big prize that some of them had been gunning for over many years; once there, they compete with their fellow students in the classroom and through comping/punching organizations and clubs, which they then seek to run; after they graduate, having a successful career (often defined by money, position and power) is the next milestone. This is what they know how to do.
There are plenty of rich kids at Harvard, and some certainly will manage their lives so as to maintain or improve their position. I don’t see this so much as compromising their principles as living up to them. I think the “compromising principles and jamming themselves into lucrative employment solely to become rich”, is something you more often see in the middle-to-lower-class strivers for whom Harvard is their ticket up and out. And plenty of rich kids are totally ineffectual at the practical side of life. I certainly know a lot of examples of all these types of people.
I don’t mean to suggest that there aren’t many, many pure intellectuals, artists and others at Harvard who stick to their principles, as undergraduates and afterward, or that things are entirely different at Harvard’s peer schools, where there are plenty of students always looking out for number one, who behave in similar ways to those Harvard grads @JHS was talking about. I do think, though, that the situation is more extreme at Harvard, perhaps inevitably, because Harvard is the top brand name in universities and therefore the brass ring so many students seek, but also because of the unique characteristics of the Harvard environment.