Biden is actually polling better than almost any Western Leader right now , so the question should be why is Trump not polling better than he is. The reason is because Trump's election denialism and scandals do hurt him but its just that general dissatisfaction is so high its not enough to defeat him currently.
I think the partisan polarization and division in the US basically prevent any president from polling lower than 33-34% regardless of how lousy their job performance is. Even Trump never came close to 20s territory on his worst day.