The fact that Bernie was who the people actually wanted both of the last couple of times and they only got scared into settling for less because of the party machine's lying fear-smear campaigns against him both times is not Genocide Joe's current problem. The fact that the party machine has also been even more effective at suppressing other Bernie-like individuals nearly out of existence is also not his current problem. Even the fact that things like that most recent video at a concert don't merely show him not dancing, but even actually show him gaping with that vacant where-am-I-what's-happening-is-it-medication-time stare, is part of his current problem but not most of it.
Most of his current problem is simply how he's been handling the actual issues that are part of the job. Even before deciding to be unwaveringly all-in on genocide, his approval was already almost as low as it is now, because he'd already spent most of his Presidency betraying us, particularly on domestic policies. There was no effort at all to keep pandemic-inspired programs that had economically helped people a lot, so people who had been helped out of poverty under Trump were dropped right back into it under Biden. There was no effort at all to pass "Build Back Better" as a complete set including the parts that would actually help ordinary people; instead those parts were only there to be tossed aside so that only the parts that amounted to corporate handouts could be passed. There was no effort at all to do a single solitary thing about health care, other than a few drug prices out of hundreds, which just demonstrated that he knows he could do more of that and simply chooses not to. There was no effort at all to really do anything about student debt, other a few demonstrations of the fact that he knows how easily he could do the whole shebang and simply chooses not to. There was no effort at all to do anything about the housing crisis or greedflation, or even acknowledgement that they exist (in fact there was open mockery of the people most affected by them for having the gall to talk about them). There was no effort at all to do anything about abortion either pre-emptively or afterward. Even outside the purely internal issues, the biggest thing that comes to mind on foreign policy that's more associated with Democrats than Republicans is how he and Congress have been stringing Ukraine along, neither staying out nor giving them enough help to actually win, a policy which serves no possible goal other than to maximize war itself, which is the worst outcome that could be imposed on Ukraine but by some amazing coincidence the most profitable one for certain American corporations that by some amazing coincidence happen to be paying most American politicians. And that's just off the top of my head at this moment, not from keeping a list all along just to make posts like this even longer.
And yes, I know the excuse, if he (or his party) had tried to do positive things, some or all of it could have failed because there are Republicans. But he/they would have at least been seen trying (and the Republicans would have been seen blocking), and that would have not only changed their image for elections coming up soon but also helped to shape the overall conversation to start shifting toward possible success in making real improvements in the future. It's better to try with some chance of failure, or even try and keep failing every time, than to be known for just not even trying.
And the biggest irony, for a guy whose entire pitch is now "at least I'm not Trump" rather than even pretending to have anything positive to offer about himself, is how much like Trump he's been. Trump always favored the megarich over the peasants, and the Senator From MBNA has always favored the megarich over the peasants. Trump was into tariffs on China, and Biden's done tariffs on China. Getting out of Afghanistan was good, but Biden was simply carrying out Trump's decision on that one (and the only difference between them there is that Biden wouldn't have made the same decision independently, so we'd still be in Afghanistan if it had been entirely up to him). Their policies/actions about the border & immigration are functionally the same. Trump would be supplying weapons for Israel's genocide, and Biden won't even consider stopping supplying weapons for Israel's genocide. Trump's servants banned abortion, and Biden isn't even pretending to care at all to do anything other than leaving abortion banned and not even being talked about by the politicians anymore.
It's not even that he's been a complete disaster in every way. He's been appointing judges who, although probably often somewhat conservative, at least aren't absurdly fascist maniacs. He's tuned down the drone strikes. He's mostly not interfered much in the increase of union activity. He's increased pay for Federal employees in a couple of different ways. But even those good points mostly have a "but" attached which I won't even bother to go into, and, even without that, a total overall record of something like 30/70 just doesn't do much to inspire voters.