There are billions and billions of people on this earth. If we forced them all to pay a tax, it would be extremely difficult to collect. People would have to climb through miles of Amazonian jungle, go into the middle of the Sahara desert, and go exploring in Arctic tundras just to collect money, which seems slightly unfeasible.
I do not believe global tax is enforceable, because not every country has the same views. More importantly, not every country is a democracy. There are different forms of currency, and there are a lot of countries that are not exactly friends of the USA. So it would be almost impossible to work with them on a consist ant basis.