Because the US has more companies that make software than anywhere else, and therefor has a greater demand for software engineers than anywhere else in the world. The supply of software engineers hasn’t kept up with the demand for their labor, so salaries go up. This has happened all over the world, it’s just more pronounced in the US because the demand is so much greater.

The situation in the US does impact salaries all over the world though. A lot of software engineers over seas work for companies that are either US based, or who primarily do business in the US.