Why are dev salaries so expensive in the US?

The US is a richer country. This is the biggest reason. People are more expensive there overall, not that they are (median) much better or anything.

The other reason is that the US corporations have managed to capture a large pie of the value created by the internet.

Supply and demand --- as always, a vicious circle.

A big factor is age discrimination. Companies prefer to hire younger developers. This drastically decreases supply.

Are salaries really that expensive once you factor in that the typical dev career is probably over by age 40? Accountants earn less but their career lasts a lifetime.