One answer is alienation. Pre 1940 politicians would personally apprentice ideological successors. Later, party machines would ensure transfers of power to popular candidates with similar views that could continue the older member’s political projects, while the older member retired and became an elder statesperson controlling party strategy and acting as an advisor.

But the Democrats and republicans have elided all political cohesion away, and candidates get in either through party stacking or via personal rhetoric.

So there are no political projects or institutional ideological continuity, there’s only the institution, blowing whichever way the loudest faction of capital says.

Thus ossification and gerontocracy as senior party members need to stay on because their machine will collapse if they don’t.

You see this in a lot of other organisations as well. We all know of the Trot org with 4 80 year olds at the top, a bunch of students who will bail after uni at the bottom, and no long term junior cadre.