That's because our government isn't a democracy. It's a republic, thank goodness. Add to that the fact that we are quite restricted in our choices, either Democrat or Republican. While we do have other parties, our system is really set up for only two. As such, one has to pick the person that they most agree with. It does NOT mean that they agree with everything that party stands for, just that they tend to agree with them more on the issues that matter most. It is certainly possible to be pro-choice AND anti nationalized health care, but there is no way to express this belief at the voting booth.

While I'm currently registered as a republican, I really lean more toward libertarian. I typically vote republican, but I am never 100% on board with their platform. They're just a lot closer to my views than the democratic party and the libertarian candidate never has a snowball's chance in hell of actually winning.