To be fair, we still don't know the loss of votes through mail-in voting. Mail-in voting is more convenient which would likely increase participation, but missing signatures, naked ballots, late or undelivered mail, etc can prevent those ballots from being counted.

When there is a major difference in voting patterns between the two political parties, then polls may not actually be inaccurate. In this case, Democrats chose mail-in voting and Republicans, by and large, chose in-person after being directed by the president.

Also, some percentage of mail-in ballots may not have reached their destination because of possible election interference through reduced post office efficiency--in particular, fewer mail sorting machines, lower hours, deprioritized mails, etc.

In short, the polls don't take into account election interference in the post office or unsuccessful delivery. If most people from both parties were voting in-person, maybe we might have had more parity between the polls and election results.