It's memes.

You're seeing the power of memes.

I mean memes in it's intended meaning. Ideas and ideologies have two relevant properties. Their ability to 'infect' minds, and the ability to spread.

The reason you always find ideologies in pars of opposites, is because this is a strong amplifier.

In the olden days, ideologies would infect societies and groups, fight their ideological wars, live and die, propagate and compete. What you end up with is things like common culture, dominant ideologies, and characteristic personalities.

In the internet age, the most contagious ideas spread faster and farther. And the meme evolves and gets stronger for each meme-generation.

Anger and hate are powerful motivators. The chemistry in your brain responsible of negative feelings is strong, and can easily outcompete your reward circuits who are designed to fire sparsely. However, you do find ideologies still who center on reward rather than punishment, but increasingly more rarely. These chemicals fuel the addiction, and a sense of community amplifies them.

Instead of people working together to improve the status quo and trend towards fixing societal issues, there are more and more individuals overreacting, and reinforcing others' overreactions.

This is true, and the reason is the infection. Anger and hate are equally present on the political left, where it's tolerated and accepted because it 'justified' - something must be done - unfairness makes my blood boil - those people will make life miserable for some other people - I suffer the pain of hurt dignity that those people suffer - the value of life is more important than economics (regardless of the complexities of any situation, certain things must be the priority or I will suffer). On the right, they're justified for different reasons, with far less emphasis on personal or collective suffering, and more emphasis on rationality (however faulty it may be) - those morons are ruining everything - we have to stop them from tearing everything down they're like mindless zombies - their arguments aren't logically coherent or rational, but they speak with such conviction, that's scary and it makes me angry - there are dangers accosted with x & y, I'm furious that those people are ignoring them - etc.

So you end up with a world perfectly divided between the two most effective brain viruses, who feed off each other, and their victims equally blind to the hateful, miserable, thoughtless, ants that they have become.