> as they worry too many new friends will diminish their own place in their friend group.

I have several friends in Seattle that have explicitly told me this.

PNW is more about not inviting, the flakiness is more of Californian thing. As someone who has spent substantial time in both places as part of a lifetime of being an expat to many places around the world the PNW and California do indeed have these distinct characteristics. Other expat friends have independently come to the same conclusions. As an expat you get to do the whole making new friends thing over and over again so I know what it's normally like and I know when it's different.

It's a generalization, which is not to say that all individuals in these locals have these characteristics.