The standard complaint is the opposite. In a generic first past the post two party system you should end up with two barely distinguishable centrist parties.
But the US system is far from generic. Instead it has several tweaks that make it tend towards extremism. The primary system is probably the biggest factor.
https://en.wikipedia.org/wiki/Hotelling%27s_law
But the US system is far from generic. Instead it has several tweaks that make it tend towards extremism. The primary system is probably the biggest factor.