There is, they just don’t like it for aesthetic and/or historical reasons.
The faction that currently runs the Democratic party is the centrist, deficit-reducing, foreign-intervention-when-necessary party of Reagan/Bush.
If the centrists and moderate conservatives could make common cause, they would easily shut out both the far left and far right wings of American politics. The demographics are there.
I think the main wedge preventing this unification is still abortion, and to a lesser extent LGBTQ rights. But it’s so weird to see two political factions that agree on 90% of policy get shellacked and overruled by their respective extreme wings. Real tail wagging the dog stuff.
These parties have primaries and Republicans are choosing—by a majority—the crazies over the “traditional” wing. They aren’t extremists. They are the party views.