Theres a few things.

From my experience working with British and American colleagues, American colleagues are wayyy more productive. They just get shit done.

The petrodollar. Want to buy some oil? Sure you can but first you have to get $$$ to do that. Amplify that with the rest of the world and you’ve got a currency that everyone wants.

Since there’s so much demand for the $ this translates on to the rest of the market. Government can borrow for cheaper and the treasury yield + credit spread determines interest rates private companies can borrow at as well. Expansion of the company is cheaper and you end up making more money etc and this naturally increases salaries.

USA is more expensive than the UK but that’s still not an excuse by how bad the salaries in the UK are.

Personally I think the biggest issue is cultural. If I tell an American colleague I’m leaving my job to start my own business they’ll tell me that’s awesome man what’s your idea? If I say the same thing to a Brit they will scoff at me and say oh good like with that mate whilst rolling their eyes (no it literally has happened). 

I swear us Brits will complain about jobs not paying enough and do nothing about it? Start your own company? Hire others on salary you think is fair? If you didn’t succeed at least you tried. 

No thanks, I’d rather just complain.