Because the USA threatens sanctions, coups, and war if other countries don't go along with them. Why do they tax citizens from other countries? Because their wars cost them an arm and a leg, and they will take every opportunity to take what they can - except from their billionaires. America is the only country in the world that taxes its own citizens when they work in other countries.

I have both German and South African citizenship, and neither of these countries have ever taxed me when I've worked in other countries.