I think a lot of it is a personality thing. Biden has pushed some important legislation through congress (not enough on infrastructure for sure, but I guess you get what you can), gun legislation (ditto WRT infrastructure), CHIPS (which wouldn't have passed if it was introduced prior to infrastructure), the rescue plan, and inflation reduction act. He's basically been hamstrung by congress, but when you compare it to Trump's signature achievement, it looks pretty good.
I think presidents get more credit and blame than they deserve. Biden doesn't have a lot of sparkle, but has done what he could. I originally supported Warren for president, but I'm not sure she could have done as much or more than did Biden.
Don't get me wrong, Biden has his faults (big time), but getting what he's gotten, when you consider the senate, has helped some. He's just kind of dull and, unfortunately, people often consider that above policy when voting.
I think the tone of his campaign is shifting and will continue to do so. I suspect as we get closer to the election, his number will improve enough to get re-elected. I know some on the left would like to see Trump win in the hope that it will cause something of a seismic shift in domestic policy down the road. It's possible that would be the case, but it's an extremely dangerous approach. I would like to see what Biden is willing to do with a functional majority in both the senate and the house, but because of the senate map this year, that seems unlikely.