I would be curious how many of them were Democrats their entire lives, and if not, when they decided to switch. While the Republicans are also guilty of making rich people feel guilty for rightfully earning their profits (as opposed to political cronies who earn their profits through regulations against their competitors), the Progressives in the Democratic party have made Guilt over the profit-motive a part of their platform, and helped to make this idea "common sense" throughout society.
What I'm getting at is - maybe many of these rich people became Democrats only after decades of being told they are evil for making a profit. Bill Gates is a prime example of this. He did more long-term good for society with his work at Microsoft than he will ever do spending his money on short-term philanthropy. Rather than striving for long-term changes to improve the state of Africa (e.g. ending the foreign aid that inevitably funds warlords, reversing the ban on DDT, etc.), he is dumping money into short-term projects that give him good publicity in the eyes of the guilt-mongers (e.g. inventing a new kind of
toilet), but will not lead to any sustainable increase in the standard of living in that region. Give a man a fish...