You may find it an absurd opinion, but I'd say it's exactly why the Democrats can't help but grab defeat from the jaws of victory.
They're simply bad at showing off the good that they do and they spend their campaign time courting some mythical moderate Republican voting base that wouldn't vote for them anyway by moving further and further right, disenfranchising their voting base. Who they then blame for their loss. Leftists don't like voting for the lesser of two evils and vote with their heart, while Republicans vote for the nominee, regardless of their positions or values. And the stats show that the Democrats failed to motivate people to get out and vote. Trump actually lost 2 million votes compared to 2020. If there's still elections in 2028, we'll see a resurgence in votes for the Democrat candidate after 4 more years of this monstrosity, but people have forgotten just how bad it really was.
The economy does better under Democratic presidents. That's a verifiable fact. Biden added 500,000 jobs to the economy and brought inflation down, started student loan forgiveness programs, and all kinds of other things. But the Dems didn't talk about that, and the media sure as hell wasn't when going to when they could blast Trump's absurd claims instead. And when we did hear about it, it was through the lens of the economy as a whole, not what you average American experiences day to day. Inflation is down, but prices aren't, and wages haven't kept up in decades. According to the IRS, 51% of Americans made $15k or less in 2021.
So when Trump says he's gonna change things, and Harris looks like it's gonna be more of the same (no matter how much hogwash Trump's claims are and things are only going to get worse), it's gonna do nothing to alleviate the grievances leftists have had with the Biden administration on all sorts of things like the genocide in Palestine. They've heard the "vote for me or it's the end of democracy!" song and dance too many times while seemingly getting nothing out of it.
One thing to note about Bernie and AOC is that they're not popular, but their policies are. During 2016, polls showed that even the majority of Republican voters supported Bernie's policies - but only if you told them what the policy was before you told them that it was Bernie's.
The Red Scare is still ongoing today, and having been labeled socialists and communists has made them untouchables and pariahs. And there has never been any Red Scare equivalent for the KKK or white supremacists, or for corporatism or anything, but there has been for socialism, unions, and anything else that puts power into the hands of the people.