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."