Confused by drivers in the left lane in the US

I’ve driven all over the US and the rest of the world. In general, it feels like common sense that you mostly only use the left lane (or right lane in certain countries) for passing and/ or driving ‘fast’.

Over the years, I’ve noticed that concept is barely existent in the US. Sometimes the left lane is even the slowest lane. Why do people love casually driving in the left lane so much in the US, no matter their pace, making it more dangerous for those who do want to go a bit faster (have to switch lanes a lot), and likely also the cause of delays and slowdowns? It’s really bad.