There are 2 differences: first, US devs are paid substantially more. Second, their stated salaries is closer to what their employer spends than our salaries. Our employers pay various taxes before giving us our stated salary, they have private insurances. This is a gross oversimplification, but that's the idea. Third, the cost of living is often higher in the US.

To really compare salaries, you need to count what the employer actually pays, and factor in the various costs of living (taxes, insurance, home, food, transportation…).