[mandude]

The primary factor being proximity to other people in the same industry is dramatically higher, and jobs within the computer industry in Silicon Valley are so much more dense than any other place on the planet. The joke in SF is that you can't walk 5 feet without running into a programmer or a startup founder. Go to anywhere else in the world and there are comparatively a very small number of programmers/startup founders.

In LA you have something similar with movies -- most people are aspiring actors and there are many people in filming and sets. However, working as a programmer in LA versus SF is very different -- there are a lot fewer engineers and entrepreneurs here per mile than the Bay Area.

It comes down to proximity to people of the same talent set. Where are the people who are good at X living? That's where X is thriving. If suddenly all the engineers left Silicon Valley, it would no longer be a tech hub.