Because your vote doesn't matter anyway, not really.
In a sense, you're right. Rarely do single-digit margins determine the outcome of presidential elections. But consider the principle behind your thinking. You've convinced yourself that your vote doesn't matter, and hey, maybe one or two of your friends do the same, and then one or two of theirs do, and now we have a plague of poll neglect on our hands, and guess what? You're complicit. You might also consider that initial reports had Iowa's GOP primary in January decided by eight votes. Point is, your vote matters. Speak up, or let others speak for you. And the really rabid ones are the ones to be afraid of.