Favorability for the party nominees on election day 2016, per RCP averages:

Trump: -21%
Clinton: -13%

Favorability now:

Trump: -9%
Biden: -1%

Both candidates start the general with lower negatives than last time, but Biden being basically 50-50 explains why he keeps leading.

Also, if Trump ends up losing, the polling that showed him still under 50% during a pandemic that's seen other country's leaders get 20-point bumps is going to look prescient.

Oh, and Sanders's final favorable rating in RCP was -8%. He was far more popular as "that guy running against Hillary" than he was by the end of this primary; he ended the 2016 primary with positive favorable numbers. https://www.realclearpolitics.com/epolls/other/sanders_favorableunfavorable-5263.html