Each citizen has only one vote. His chances of breaking a tie on election day are considerably smaller than his chances of getting killed in an auto accident driving to the polls. People operate according to incentives. Think how you would approach an election if you knew that you would be casting the tiebreaker. Most people know that breaking a tie is highly unlikely, and they act accordingly.
Rational ignorance is intrinsic to voting.
Your one vote won't be decisive. In most other contexts, your choice is decisive. When you go to the store and select an item, you get that item. When you go to the polls and vote for Smith, you don't thereby get Smith. You may get Smith, but not because you chose him.