He actually said on camera that the reason he is running this time is because Trump is running, meaning that if Trump wasn't running, Biden wouldn't be running, meaning he doesn't even really want the job.
The DNC put up a guy that doesn't even want to be president again, and now 40% of democrat voters want a new nominee because apparently they weren't paying attention for the last 4 years and only realized now, during the debate, that Joe Biden is dealing with cognitive decline.