I don't understand that old argument "you didn't vote so you don't get to have an opinion!" I don't vote because I have an opinion.
I think people think of voting too much like buying. It would be pretty bad form to walk into someone else's house and say it was ugly, or to get into someone's car and say it smells. Perhaps people think of "their" politician in the same way.
Alternately, some people, formerly including myself, may see the whole thing as a tug of war. "If you aren't pulling your weight on one side of the pit, how can you stand there and tell me I'm wasting my time?" Of course, once you figure out that you don't get any cookies if you win..
EDIT: ... Even more...
If politicians can put their hand on the stove and not get burned, why should they stop?