Typically, when customers are unhappy, there will be a new entrant (Five Forces of Competitiveness). The majority of voters polled say they would rather an independent option. But the current political landscape doesn’t see the unmet need being addressed.
In fact, The duopoly works well together to insure competition is kept out
The duopoly doesn't compete for voters
Ref:
Democracies require trust. Most of all that power-holders won’t abuse their dominance.
But trust has collapsed recently - in America and more broadly. since obama, trump and biden, trust in government is below 20%. 1 in 5 believe “army rule is good”. 85% of Americans feel the country is headed in the wrong direction The political duopoly doesn't address customer complaints. this general sentiment extends to media, science and general “expertise”.
Competing for the middle would be destructive to both parties. Ignoring them is safer. Besides, what can an unhappy voter in the middle do? Because The political duopoly doesn't address customer complaints, the middle voter has no appeal.