because their interests don't allow it. Their donors and the people they work for, recruit from, and get hired by after leaving office, are all the billionaire imperialists that benefit from exactly what the democrats are doing and have been. It's in the base structure of the democrat party. A lot of times these capitalists donate to the democrats, then vote republican. Both parties work for the same class of capitalist imperialists in whose interest they are so entrenched, they literally can't do anything else but fly to the right and become more and more indistinguishable from each other as they both compete for who can be better at committing genocide and who can be more fascist on immigration and the border concentration camps and also militarily and operationally abroad

The only way to break the duopoly is to throw weight behind a 3rd party (the further left the better, PSL or greens if you're a liberal or your state is strong for it and you like that idea), which would starve the democrats of the margin they need to ever get power, and force a reorientation where the ruling class would have to float a reformist "labor" party to keep people from further radicalizing and flooding to the socialists away from the open-fanged republicans, who would remain as the only real political force when the mouth-closed-smiling but just-as-fanged democrats, entrenched in their position, can no longer cruise-control on "not being the republicans." Which they already are in most ways, and in the ways they wear a mask of not being they're totally feckless and actively capitulate in order to drum up more fear about the republicans to scare people into voting for them without doing anything to earn those votes (Obama had both houses of congress. He could have codified abortion rights and LGBTQ civil rights protections into law then and there. He didn't. He also gave away a supreme court seat. And Biden has not only not forcefully pushed to expand and pack the courts, but has actively denounced the idea as "politicizing the courts" as if that ship isn't already past the horizon. And has done fuck-all to stop the book-burnings, anti-LGBTQ laws, criminalizations of abortion, etc that are currently happening under a democrat president. They don't care about any of us and never will, and it will continue to get worse under the duopoly).

The ruling class floating this "labor" party would itself cause the democrat party to split in half, with half hedging their bets and pouring into the 'labor' party and the farthest-right establishment remnants stopping pretending they're anything else and joining the republicans. Which would then "democrat-ize" the "labor" party and alienate their left wing who were trying to escape those same people and interests into joining with the socialists. This is why I say throw weight behind a socialist 3rd party. Because then you actually have a growing counterweight pushing forward against this rupturing contradiction and highlighting how badly these people play politics, as the establishment and their bourgeois politicians are scrambling in retreat to reorient and reconstitute politically, having obviously grown so entrenched and corrupt and complacent they've forgotten how to play politics.