The industry is a lot "closer" than people expect; it runs a lot more on handshakes and people going to the same parties than on cold numbers, so not only do startups have to be in the US they have to be specifically in SV.

Two decades ago at a UK chip design software startup, the founders started a US branch and migrated there for just this reason.

(The SVB bank run shows how strong the "being in the right whatsapp group message chat" is)