Joe Biden seems like sort of a "Goldilocks candidate" in the sense that he is not too far to the left, and not really on the right, at least not anymore. There is a phenomenon called the "rally round the flag" effect, which would seemingly benefit Biden because of his handling of the pandemic, which was a crisis.
There are some moderate Republican governors in some of the most heavily Democratic states in the country who have taken liberal social stances but more pro-business as well who have insanely high, Bush 9/11 level approval ratings. What makes this even crazier is polling consistently shows a number of them are more popular with Democrats.
More pertinent to the discussion is that some presidents had very high and thus bipartisan approval ratings, at least during certain times, like Bush did in the months following 9/11, Obama during most of his presidency, and many others. Why hasn't Biden been able to get a "rally round the flag effect"?
Note: I know that this sounds like an opinion question on the surface, but it is not intended that way. It is based on factual contexts, such as rally round the flag among other things. I also say so because he has characteristics that have led to high approvals in the past: moderation, desire for unity, and a crisis (COVID-19).