The real reason is that politics, and especially the two party system in most Western countries, is based on polarization, e.g. blaming the other party for all the evil of the world.
It's not some abstract "politics". For left-leaning, it's about freedom for women to decide about their own body, about respect towards minorities and people coming from other countries, just to name a few. For right-leaning, it's about protecting families, cultivating the tradition, prosperity of the country, the right to defend oneself etc. Politics became almost a new religion.