A lot of people here are giving various answers, and that's for 2 reasons. You asked a very loaded question in a lemmy instance that started with an explicitly socialist bend, and you're essentially asking people about very personal reasons they hold a political belief.

Everyone's political beliefs are shaped by the conditions of their life. For example, I grew up poor on a rural farm in the Midwest of the US. Over time due to various things, I had several life situations impact my views on politics and the world. I started to learn things, things that didn't make sense, things that challenged my world view, things that I knew were wrong but didn't know why they happened. Eventually, I rejected liberalism and needed to find something else. That something else came in the form of a walkout at my workplace. I was thrust into the labor movement. Now I'm an anarcho-syndicalist (I believe all hierarchy is bad including capitalism and governments, and the people should govern themselves through unions and other forms of organizing).

This is a very, very brief description of my life from when I was born to literally right now, and how it impacted my beliefs. This process of life impacting personal political beliefs is called our "material conditions". People may have similar material conditions and completely disagree, or drastically different material conditions and agree on everything. More and more Americans are seeing and feeling the dramatic impacts of capitalism and the power of a few people on the top. That is driving people both to the anticapitalist left and to the fascist alt-right. Whatever reasons you read here, know that they're justifications for their material conditions causing them to take a radical position.