Big, well established companies play a big role in seeding the talent need to create startups. Before it was called Silicon Valley, the San Francisco Bay area was already home to big defense contractors who employed lots of engineers. Boston has for 300 years been a center of not just intellectual activity, but also commerce. Austin, in addition to Dell and National Instruments, has such non-local tech players as IBM, Intel, Freescale, Sun, Apple, Samsung, and many others.
In addition to providing jobs to local tech grads, these companies also recruit (inter-)nationally drawing to their cities engineers and scientists from around the country and world. Such recruiting counters the natural forces that cause a large fraction of local grads from any university to take employment in some other city from the where they got their degrees.
Concerning the H-1B issue, "big, well established companies" also provide a soft place to land for foreign born talent seeking the means to stay in the United States after their student visas expire upon graduation. Immigration law is the unnatural force overlooked in the Pittsburgh case. Promote a more favorable immigration policy and the Microsofts will follow.