Because America is the only country in the world that insists on taxing its citizen when working in another country. No other country does this.

I am a dual German/South African citizen living in Portugal. I am writing for Medium. Medium takes off 10% because I am working for an American company. If you were living in America and working for a Portuguese company, Portugal would not charge you tax, but America would.

In all other countries, tax is only charged in the country where you live.