Maybe these offices are just following the great standards set by the people working at the HQ.
Even then, there are a bunch of companies that have their engineering based in other countries.
I personally worked for a few companies that are major player in their fields (although not unicorns) that have offices here in the Greece.
Most of these companies have one thing in common, the leadership, everyone at the C level, work and live in the US. In fact, the companies I worked for had HQs in the SV.
I don't think SV has a significantly better talent pool. My anecdote is that my local team has had more than a couple laughs looking at code written by our SV-based colleagues.
But I think that building your business there certainly gives you a huge advantage.
Your engineering doesn't need to be there.