Yes, I personally would prefer to redistribute internationally (and hence that's where all my personal donations go) but from conversations I've recognized that it's politically unpalatable to the vast majority of Americans. The vast majority of people somehow think that their fellow Americans are more deserving than the international poor (and will quite strenuously argue that things like having toys for local kids is more important than literally saving lives).

The nice thing about our tax system is that you can write off donations. So even if we did adopt stronger tax measures, billionaires could still choose to support more effective causes than the government.