There is not enough time to change the candidate. Besides, it used to be easier to do this (before 1982 I think) - the decisions were made by party barons, not delegates, as is the case today. Two, there is no obvious candidate to replace Biden: Harris is less popular than him and Michelle Obama doesn't even want to run - it's hard to say whether she has the competence and experience to do so. From the Democrats' point of view, it`s best to keep Biden, who will probably lose and get ready for 2028 with a new candidate - Trump can only rule for one more term, because the one from 2016-2020 counts.