Well beyond just the jobs, the networking, etc...the Bay Area does have some intangible non-tech-related amenities that are hard to find elsewhere

Look at an aerial map of the Bay Area and take a gander at the hundreds of thousands of acres of incredible open spaces. These are all within a short drive of many of the residents. I can go from Palo Alto to old growth redwoods in thirty minutes on a good day! Note that this has nothing to do with tech.

Also, rich people like to be around rich people. There are probably more billionaires in a couple of blocks of Professorville (four that I know of) than there are in the entire state of Michigan. Thats why people are in Palo Alto and not Ann Arbor.