I don't know why it isn't the case in places like the UK tbh. I know some countries have things like national proportional representation, wherein a percentage of seats is allocated to each party based on voting percentage, but I'm not sure if those are common in Europe or just Oceania.

It's because we have one huge Presidential election in addition to FPTP that sucks all of the oxygen out of the room. And it's not that you have to win a plurality of votes, you have to win a plurality of a plurality of votes within each state. This requires a much larger infrastructure that makes it easy to turn toward down-ballot races in off-years.

Also, the nature of America's two parties is that they are historically sectional rather than ideological. If you wanted to run as a social democrat then it's not the case where you need to advance some socdem party. You just run from within whichever party you represent.