The rich don’t earn money from an income. They earn it from investments. We tax investments at the same rate as the top bracket (37%). If we raised the top income bracket to 38%, it would push more rich to receive their income via investment. However, raise the investment tax (capital gains), and we drive foreign investment away. A lot of foreign money is in America because we have a long history of stability and a low possibility of the people rising up and nationalizing ownership of foreign property. Drive that money away and everyone suffers, but that also makes it impossible to raise taxes on the rich.