This is a matter of math and political mechanics, not statutory or constitutional law. The framers of the US Constitution mostly didn't even want parties to form, but parties formed almost instantly regardless. Then the reliance on single ballot plurality voting (first past the post) led to two parties. Even if a party has significant internal fractures like you find in both the Democratic and Republican parties, an equilibrium of two roughly equally sized parties is held because any other substantial party is pressured into a merger to produce an electoral majority. There have been several iterations of the two party system and every time it produced that equilibrium again.

[–] yogthos@lemmy.ml 2 points 2 years ago

The whole US system is basically an oligarchy. Here's what a recent large scale study analyzing many decades of US policy concludes:

In the United States, our findings indicate, the majority does not rule—at least not in the causal sense of actually determining policy outcomes. When a majority of citizens disagrees with economic elites or with organized interests, they generally lose. Moreover, because of the strong status quo bias built into the U.S. political system, even when fairly large majorities of Americans favor policy change, they generally do not get it.

I also recommend Democracy for the Few which is a great book discussing how US political system actually works. Any semblance of democracy is pure theatre.