Because its proven fact that mail in votes are easier to cheat so if the democrats have the fix in they need the % of legit mail in votes to break in Biden's favor. That way you can add in an extra 10 million votes easier.
When DJT tells his supporters, "Don't mail in your votes, go to the polls!", and Joe tells his supporters, "COVID is scary, stay home and mail in", why is it so hard to believe that the mail in votes are going to overwhelming go to Biden, and if those are counted after the in-person votes...