My perspective as a hiring manager at a UK based SAAS company is this:
I will and do pay as much as I can afford to secure developers who are actually good at their job. I can't really comment on why they're so hard to find, but my experience is that they really are.
Maybe it's the side effect of there being very few of the kind of developers who could command those SV level salaries in the UK, or maybe they just aren't in the market for my kind of opportunities. It's a hunch that would require more data because I've not hired developers in the US so I don't actually know if there's a quality difference.
Anyway, I'm hiring at the moment.