The Netherlands has proportional vote, that’s why.
With electoral circles instead of PV, mathematically the two largest parties get way more representatives than the percentage of the public votes they get, and the bigger the electoral circles and fewer the representatives the worse it gets.
(Further, voters own behaviour changes to one of “useful vote” rather than “choosing those who better represents them”, plus tribalism becomes way more extreme when there is only a black & white choice - so lots of votes are driven by team loyalty - all of which makes it even worse)
(Also smaller parties dissapear, both because they can’t secure funding and because their members lose hope of ever making a difference. The closest you get to “small parties” in the US are independents, running for a very specific electoral circle only and whose voice is a drop in the ocean in a place like the US Congress)
The US has single representative very large electoral circles for Congress and double representative State-sized electoral circles for the Senate, so their system is rigged to pretty much the max it can and the result is a power duopoly.
I lived in The Netherlands and now I live in a country where the system is somewhat less so (smaller electoral circles, multiple representatives per circle) and even here you see the two largest parties getting and extra 10-20% each representatives in parliament compared to the popular vote (the governing party has 56% of parliamentary seats on 42% of votes cast) whilst the smaller parties have half as many representatives as their popular vote (in other words, every vote for a smaller party counts less than half as much as a vote for a large party, which is hardly democratic).
Most so-called “democratic” nations have this kind of rigged system, but places like the US and Britain take it to the extreme, so it’s unsurprising that when the economic supercycle is at the point where the many start hurting, in the absence of true choice you get instead the internal takeover of the rightmost of the party dupoly by the Trumps and Boris Johnsons of this world offering an ultra-nationalist far-right populist mix of othering, scapegoating and simple “solutions”.
(Funilly enough if you compare The Netherlands with Britain, whilst even now the far-right is stuck at maybe 20% in the former, in the latter it took over the Tory Party from the inside - which is far easier than convince half the population to vote for them - and hence has been in power for almost a decade with an absolute majority).