It seems like there must be some opening there for companies willing to pay 25% more to poach great talent.

It just doesn't work like that in the UK, even FAANG pay peanuts because they can get away with it.

But why can they get away with it? Is there a glut of talent? Or a market full of developers who have zero interest in making more than $100k?

There's just no other options; it's still one of the best markets in the EU and going to the US is impossible for most developers so they have to accept the lower salaries.

You would think that some startup in NYC would decide to start hiring in London, then. Instead of struggling to recruit senior developers with a middle of the pack NYC salary at $125–150k, they could offer the same salary in the UK and be standouts in the market.