Short answer: No, all politicians aren't greedy.
Long answer: Politicians are people, like anybody else. The benefits awarded to them for being politicians rarely outweigh the responsibility and stress they endure. Try putting yourself in their shoes (and I understand that being in their shoes would probably be highly preferable compared to being in the shoes of your average African citizen), and imagine yourself in front of millions of people expressing an opinion which almost certainly around 50% of them will disagree with. Or imagine being offered money CONSTANTLY to simply lend your support to an idea or company. Imagine if every single thing you said was praised by thousands or millions of people, but also hated by just as many, no matter what you say.
Would you be able to spend a lifetime turning down large sums of money for 8 seconds of work? Would you be able to handle the criticism of opposing parties (there are ALWAYS opposing parties) and even personal attacks from the press and everywhere else? Would it really be worth it?
Politicians are people, my friend, like you or I. A lot of the time, it isn't even money that is a factor in their decision making, but instead personal bias or ideology. For example: Most politicians (in the States) who are against gay marriage or abortion, are against it because of their religion, or some personal experience. There isn't much money to be made in either issue. Personally, I think they're disgusting human beings for this opinion, and I attack them constantly. But they have a whole lot of support as well, and my side gets attacked constantly for our position that abortion and gay marriage should be legal.
Some politicians are revolutionaries, but they are usually revolutionaries who became politicians, not vice-versa. But even these politicians can be met with serious resistance and held in contempt by millions. There is no way to win.