The framework has a lot of fat on it. Tens of thousands of bureaucrats and lobbyists and party cronies exist to facilitate the corporate-financed bipartisan system. To change the framework, you’ve got to fight with all the folks who profit off their positions in that framework.
The parties have too much power over the pick because what are the voters going to do
Parties have enormous amounts of money and manpower at their disposal. The money comes from business interests exchanging cash for political favors (a thing which SCOTUS now recognizes as perfectly legal). The labor comes from armies of paid workers, complemented by even larger seasonally sourced pools of local volunteers, who have been whipped up into a panic by mass media.
That’s what gives party leadership the influence it commands. There are huge carrots for folks who play ball with the sitting administration and huge sticks waved over the heads of party-splitters and indie candidates.