Since nothing and no one is devoid of either, all is both, and appears gray, but we must remember always that our choices remain only black or white.

I think it largely a symptom of the two party system, which encourages left and right to play to their base for the sake of turnout (despite partisanship driving GENERAL turnout ever lower as the process disillusions ever more people.) For the electorate as much as the elected Jim Hightowers quip is more true now than ever: The only things in the middle of the road are yellow lines and dead armadillos. That can be simplified as "everything in the middle of the road is either dead or yellow." Moderates and Independents are much discussed, but that is because roughly 40% of the country is always precommitted to each side; as long as they show up on election day they will balance out and make the remaining 20% the decisive factor, but reaching out to that 20% too much runs the risk of alienating the 40% who MUST show up to make victory even possible. That usually leaves moderates and independents seeking the least radical candidate, because there is none that truly represents them; in a two party system, no candidate can afford that.

So both sides demonize each other as radical extremists, both to energize their own base and ensure moderates are terrified of their opponents. The result is a truly vicious circle in which politics becomes ever more polarized: I say you raped my dog; you say I raped yours after I killed it, etc. etc. Small wonder more people than not skip every election save presidential ones.