I'd lean towards the "networking" explanation as the strongest factor among those you posited. Other schools in the country may provide just as high quality education, or even better in certain fields, but Harvard is still the most recognized name in higher education in America, and has a long history of prominent alumni associated with it. It's an extremely good place for some of the brightest minds in the country to meet people who can get them advantageous placement to make use of their abilities.
Plus, having the most recognized name in higher education is a very valuable signaling asset. People in pretty much every line of business in the country associate a lot of status with "Harvard graduate."
[Added: Solipsist made good comments that partially account for the phenomenon, and Douglas_Knight pointed out that the figure of $200m+ below should be $30m+]
This article reports that Harvard has 2,964 alumni worth $200+ million, with a total wealth of $622 billion. These figures are staggering:
Note that Bill Gates "only" has ~$75 billion and that Mark Zuckerberg "only" has ~$30 billion, so that they don't account for Harvard's decisive advantage over other universities.
What is going on here? Why would Harvard come out ahead by such a large margin? Its acceptance rate is smaller than those of Stanford, Yale and Princeton, but by no more than 25%. Moreover, this has been true historically
One can ask a similar question of University of Pennsylvania, which is significantly less selective than Yale and Princeton, but has a decisive advantage over them.
Some possible explanations for the discrepancy:
Regardless of which of these explanations hold, there remains a question of why they would hold.
Is Harvard a better choice than other universities for students who aspire to be wealthy than other top schools?