I am more than ready for my thoughts to be considered economically naïve, but I wonder if it's to do with the ability for labor to move?
In the US it is easy (from a legal POV) to move between California and Texas or vice versa so an American employee would probably move to California if a much larger salary depended upon it. If an employer doesn't really care if someone is in any particular location within the US, they may nonetheless feel happy paying what they valued the employee at if they did force them to move to California, say. Of course, they may also attempt to pay less, but that's a risky strategy since the employee could just as easily then request to move to the expensive location.
The UK worker earning 1/4 the wage does not have this freedom. Very few UK workers can move to the US (or any other economy than Ireland) and so the market rate for developers in the UK is very specific to the UK. Only a fraction of the best developers who both want to leave and be able to leave. There's a certain local demand and supply and the salary has found its natural resting point, protected by a moat of visas and regulations.
In the US it is easy (from a legal POV) to move between California and Texas or vice versa so an American employee would probably move to California if a much larger salary depended upon it. If an employer doesn't really care if someone is in any particular location within the US, they may nonetheless feel happy paying what they valued the employee at if they did force them to move to California, say. Of course, they may also attempt to pay less, but that's a risky strategy since the employee could just as easily then request to move to the expensive location.
The UK worker earning 1/4 the wage does not have this freedom. Very few UK workers can move to the US (or any other economy than Ireland) and so the market rate for developers in the UK is very specific to the UK. Only a fraction of the best developers who both want to leave and be able to leave. There's a certain local demand and supply and the salary has found its natural resting point, protected by a moat of visas and regulations.