All this is true for political parties in general, but the identity politics factions in the Democratic party make things worse than usual because by nature they can only see things only through the lens of their own grievances and are incredibly self-centered and are kept together mostly by hate for the majority. Also, there are a lot of them.
So, there's a ton of things you can't do. You can't piss off any single large faction or its leaders too much, or they'll bolt, with consequences as described above. You certainly can't piss off all the factions, or they'll remove you. You can't bring in an outsider, no matter how good their chances of winning may be, because it will piss off everybody who's spent decades kissing ass and working their way up the party ladder. And nobody in the upper ranks will ever sacrifice their own career for the good of the party, because those sort of self-sacrificing people won't have a career long enough to reach the upper ranks, and they can't parachute in from the outside either because of the problems above. If anyone is apparently self-sacrificing in party politics, it just means that they've been secretly promised something good later as a reward for their forbearance.