Civics education is bad to nonexistent so they do not learn about the spoiler effect, let alone the inevitable math behind first past the post elections. People vote based on how they think the system ought to function instead of how it actually functions.

Because the US first past the post system ensures there's only two parties. So unless you can threaten one, you don't have input.

As a non-american, you aren't close enough to notice that the people who say this are taking advantage of the situation and have been for decades.

Because it’s logically stupid and ushered in fascism.

As a non-american, here in my country leftists vote for many different parties, each too small to actually pass the threshold