The US has had many very successful tech companies. Those attract talented engineers that want to work there. Partly this is about supply and demand but the other part is that engineers who work for the successful companies see their compensation go way up due to stock options or RSUs (which are/were offered to attract said top talent and motivate them to work hard). If you have a new AI startup (for example) and you want to hire top talent away from Google or Microsoft or Apple of nVidia you're going to need to offer a competitive pay.

So competition over a limited talent pool in combination with a vibrant economy and many successful companies that make a lot of money. At least historically.

High salaries isn't limited to tech. Bankers, lawyers, doctors e.g. tend to earn more in the US.

Another aspect is that you get paid more but you potentially have higher expenses. So some of the delta is related to cost of living. But not all.

Well, for a while, the USA was probably THE premier country for electronics and IT. As a natural result, companies offer top dollar for top talent.

Now days, companies offer top dollar for Junior-level AI talent. And you see where that's going.

Americans are individualists and value their own career higher than any "loyalty" to an employer. So employers in fast moving sectors have to compete for talent with the money they offer.

The standard answer is agglomeration effects, I think.