It's enough to think for a couple of seconds to realize it has nothing to do with population density, otherwise you could freely discuss politics in less populated areas of the States.
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.
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.