> Yes, but why does the US have a structural duopoly.
Because of the electoral system.
> I don't think it is close to provable that the US's system of government innately requires a duopoly.
The structural incentive to duopoly in FPTP voting is mathematically provable.
> First of all the US has a two round system in most states (most states are open primaries).
Open partisan primaries, which is what most states with open primaries have, aren't the same as a two-round system in terms of promoting multiparty democracy (in fact, they just further reinforce duopoly, since they give voters not aligned with the major parties a chance to influence major party candidate selection instead of participating in alternative parties, which opportunity has a higher expected value when the general election is by plurality.)
That said, France is, nevertheless, the one major example that superficially seems to work.
OTOH, despite having multiple parties, it's also the established democracy that has empirically nearly as bad a problem with ineffective representation and dissatisfaction with government as the US (see, Arend Lijphart’s Patterns of Democracy), so while it manages to paper over the partisan duopoly symptom, it doesn't actually deal with the representational problem.
> And some of us have actually lived under the electoral process and politics of other countries
So have lots of people in America, and outside of America, including many of the people who have literally written the books on how the US electoral systems causes the both the narrow dimensionality of political discourse, and the partisan duopoly, and the representational failures of the American system of government.
Because of the electoral system.
> I don't think it is close to provable that the US's system of government innately requires a duopoly.
The structural incentive to duopoly in FPTP voting is mathematically provable.
> First of all the US has a two round system in most states (most states are open primaries).
Open partisan primaries, which is what most states with open primaries have, aren't the same as a two-round system in terms of promoting multiparty democracy (in fact, they just further reinforce duopoly, since they give voters not aligned with the major parties a chance to influence major party candidate selection instead of participating in alternative parties, which opportunity has a higher expected value when the general election is by plurality.)
That said, France is, nevertheless, the one major example that superficially seems to work.
OTOH, despite having multiple parties, it's also the established democracy that has empirically nearly as bad a problem with ineffective representation and dissatisfaction with government as the US (see, Arend Lijphart’s Patterns of Democracy), so while it manages to paper over the partisan duopoly symptom, it doesn't actually deal with the representational problem.
> And some of us have actually lived under the electoral process and politics of other countries
So have lots of people in America, and outside of America, including many of the people who have literally written the books on how the US electoral systems causes the both the narrow dimensionality of political discourse, and the partisan duopoly, and the representational failures of the American system of government.