1. To be fair, many people felt that President Obama was what this country needed. If it had been legal, he would have easily won a third term.
2. Vice President Biden is a very old and confused man, but many Dems feel that he can beat President Trump. So they are supporting him. I have just read that he is doing very well among a certain demographic (the one that turned out in huge numbers for Candidate Obama).
3. The Dems have only one goal: To kick out President Trump. In any case, they realize that a President Biden would be a figurehead, with the government being run by eager young progressives.
4. If there is a debate between President Trump and Candidate Biden, the former would be able to totally humiliate the latter. But who knows? Maybe President Trump would decide to earn praise by laying off the old, pitiful guy.