Because Silicon Valley companies derive all of their revenue online and from the digital product experience.
Every improvement that:
AirBnb
Uber
Lyft
…make to their products, means more revenue.
So they hire the best talent they can afford. And they can afford a lot.
The hivemind of designers, developers and product managers at Yahoo learn best practices, then move to Google, then Facebook, then Pinterest, then Uber, then AirBnb.
Sometimes they teach these practices at conferences.
But many keep the cards close to the chest.
Not all of these have to be in Silicon Valley, but there’s a strong hub.
A counterexample is Zola – but that came out of the Gilt and Fab ecosystem.
Or Snap in LA.
Or Shopify in Ottawa.
Or Fiverr in Israel.
The point is, there’s a strong hub in Silicon Valley. So if you go try to hire a product designer that has never worked at one of those companies. And they don’t have them in their portfolio. Well you’re really not going to get the exact same level of quality.