Trying to answer this would fill (and almost certainly has filled) numerous Ph.D dissertations.
There are a multitude of reasons. In no particular order:
* The utterly broken and ruinous US Senate, whose composition would be unconstitutional were it not written into the constitution[1], enabling a tiny minority of the country to block any meaningful federal progress on a host of issues
* The US's strong mythos of the Protestant Work Ethic, which leads many people to believe that people succeed or fall on hard times due to merit rather than luck
* Newt Gingrich, who in the 90s introduced hyperpartisanship to Congress, turning a body where members of different parties were friends and had good working relationships into a zero sum game
* The fact that one of the two major parties campaigns on "government doesn't work" and as soon as they're elected to their best to turn that sentiment into reality
* The impact of greed in the US and its successful capture of the media and significant chunks of regulatory apparatus
* The utilization of that media control to push divisive narratives that pit the lower classes against each other instead of focusing on the real problems and their causes
* The goldfish-like memory of too many US voters who buy into narratives like "they're all the same" or get frustrated when one party can't fix everything in 4 years and elect the other party - paying no heed to the fact that building is much slower than destruction or the obstructionist tactics.
That's just scratching the surface.
[1] Seriously, the Supreme Court has ruled it unconstitutional for any legislative body to be based on land instead of population. https://en.wikipedia.org/wiki/Reynolds_v._Sims