10 million people who voted for Biden, but are you sure they were Democrats? Just because a "Never Trump" Republican voted cross party in 2020 doesn't make them a Democrat. To me it seem like the majority of those 10 million missing votes are Republican and Independent voters (along with some progressives) who chose the couch this time. It looks to me like Harris did a reasonable job at turning out the Democratic party base.
@antares @Bellison22 I disagree but perhaps someone will research it more and find out.