I'm actually receptive to this argument, but there are two problems. One is that we have no mechanism for implementing international policy of this sort even in theory (i.e., there is no jurisdiction with authority over both those to be taken from and those to be given to). The other is that, absent such a mechanism, most attempts to implement such transfers will likely increase inequality, because, for instance, the elites of whatever country we want to benefit will simply appropriate all the intended transfers. In other words there is no even remotely reliable way to convert, en masse, cars in my country to bicycles in a poor country somewhere else.

This makes it unwise to attempt to do such transfers on a large scale. That said, I would support something like a tax whose revenues are specifically directed towards improving the lives of much-worse-off people elsewhere in the world in ways that are carefully chosen so as to be less vulnerable to graft. (These would likely be in-kind benefits like latrine construction, etc., rather than monetary grants.)