Sure, a lot of successful tech companies come out of Silicon Valley, but there's a disproportionate amount of tech companies in Silicon Valley to begin with. It's certainly not obvious that the success rate is any higher there.
You could use the same argument to "prove" that the hub of any industry is not a particularly good place to do that kind of work. By the same reasoning, LA is not a particularly good place to make movies, or New York to run a hedge fund.