Well, they can't in time for the election, anyway. There's an old saying in politics that "personnel is policy" which refers to a lot more than just having someone competent in the job. It's a reflection that politics is about coalitions - building them and maintaining them. The coalition members get their cut of the government largess, and pay for it with loyalty to the guy at the top. If they're not loyal, he gives them their pick slip and they lose the largess.
This was actually Trump's biggest mistake when he was president, not filling the Federal Government with his coalition. In his defense, he was in the middle of a Republican civil war, where there were multiple factions and multiple coalitions.
That's exactly what the Democrats face now, and why they can't put Humpty-Dumpty back together. Because there are multiple coalitions, whoever emerges on top won't know if he (she?) can trust these coalitions because they aren't his coalitions. They might be able to be integrated into his coalition, given time, but time is exactly what the Democrats do not have right now.
It takes time to forge a governing coalition - just look at any parliamentary system: when the government is stable it is because the governing coalition is solid. Ministers can issue policy with a reasonable expectation that it will be supported and carried out by the coalition members. When the governing coalition is unstable, chaos results. Orders get ignored or slow walked or subverted because the Minister no longer has the loyalty of the coalition members.
Eventually a leader emerges who can attract key talent from outside coalitions and integrate it into his. This will involve rewards like positions in the bureaucracy or some such - featherbedding is the name of this game. But until this all gets sorted out and the new coalition is filled with people who think they're better off with the new leader than without, nothing is going anywhere.
Even worse, there will always be serious back stabbing between different coalitions. Trust is not a virtue most politicians hew to, and quite frankly until they are in a position to remove perks as well as give them, they would be a fool to trust just about anybody.
Some day a leader will emerge to stitch together the various coalitions that make up the Democratic party. It won't happen in the next 100 days, sure as God made little green apples.
The biggest implication of this is that it will be much more difficult for the Democrats to "fortify" the upcoming election via 2020-style shenanigans. Sure, the party bosses will want to, but how much do they trust the other coalitions to support them? Would other coalitions even go so far as to rat them out (with plausible deniability, of course) - leading to various party elders behind bars. That certainly would make it easier for other party elders to construct a winning coalition once they've taken out some of the competition.
Like I said, these people would have to be fools to trust very many people, and an election cheating scheme requires a lot of people to pull off. When everyone is on-side you get the 2020 election. When lots of people are very much not on-side you get, well, the Italian government which has had something like 60 Prime Ministers in 80 years.
The best analogy I can think of is the scene from The Godfather where all the families get together to divide things up. Nobody trusts anybody. That's where the Democrats are right now.
I repeat: you can't put a coalition together overnight - heck, it's taken almost a decade for Donald Trump to put together a serious coalition and a lot of his party still hates him. I think that the Democrats will come more apart before they start to come together as the various factions start putting out mob hit style rumor whispers about their Democratic competitors. We will hear a lot about this in the next few weeks.
And this is why the only choice at all for them to to fall in behind Kamala and hope for the best in the down ticket races. But remember, while Kamala might have inherited Slow Joe's campaign cash, she was never really part of this coalition. It's not loyal to her at all. It may be that she's been so ineffective in office because Joe's coalition kept sabotaging her. She has to build a coalition, and right quick. The cash will help her there but coalition building takes time.
She doesn't have that. What she does have is a whole boatload of enemies in the Democratic party. Some of these think that their best bet to get to the top of the greasy pole is for her not to get there. They'd rather have Trump in the Oval Office because they will have 4 years to build a coalition. If Kamala is there, things are a lot trickier for them.
I almost feel sorry for the Democrats in general and Kamala in particular. Almost. It's ironic that all their short term tactical maneuvering has led them to this very spot. Couldn't have happened to a nicer bunch of Mob Bosses.