Power play, here in Oz there is no real difference between the parties in our duopoly. That seems to enable/encourage the extremeists in each to try and draw the fringe voters.

I'm against extremism in all forms. A good life is about Balance and Moderation.
That could be the most hippy thing that I've ever said.

Is it just politics, or could it be all of humanity?

Omg, just listening to Trump talk is enough to send me to crazy land. He is the best material for comedians. You never know what name hes going to call someone. Totally inappropriate. His tweets aren't any better. He has caused a big rift because he is so far to the right. He can't stand LGBQT people, people who are black or brown and immigrants. His policies on education, environment and religion ( not everyone is a Christian) taxes and the military are wrong. I don't care what Trumpies say, he is wrong as president. Ohh I didn't even mention Stormy Daniels and his making money off his businesses...

The more things change the more they stay the same.

There's always been extremism in politics. Those engaging in it just have more platforms
to exhibit it now.

In the US? Because voters became apathetic

A couple of reasons.
Conflict is good TV. Calm people generally get on the tube. You want to noticed, you better throw bombs.
Districts are gerrymandered, so it is a candidate's interest be extreme right or extreme left, to appease the primry voters.
Members are now expected to be in their home district every wkend, mostly to raise money. In the old days, members actually socialized with each other, including across party lines.

Those are gut impressions and not based in any research on my part.

Watergate is the most event in my life. If Nixon was no president, would he get away with all those things?