"EV company owners" aside, most voters in this country just want something to change, and they'll vote for whoever promises the most of it. Harris' campaign didn't do anything nor promise anything that resonated, and practically everything she said ended up morphing into her highly-rehearsed stump speech. No talks about Medicare for All, no talks about the minimum wage, no talks about legalizing weed, and kowtowing to the right on border policy by accepting the 'crisis' framing. Harris also failed to address the situation in Gaza in a way that mattered, even though it was a major issue for undecided voters in key states like Michigan. Over 100,000 Democratic primary voters there cast an uncommitted vote over Biden's handling of Israel and Gaza, which is more than the margin by which she lost the state.

The right took advantage of this. An EV company owner paid a PAC to distribute ostensibly pro-Harris pamphlets in predominantly Arab neighborhoods in Michigan saying she was the most pro-Israel candidate on the ballot. The right helped put abortion rights directly on the state ballots as propositions, letting people believe the choice could be separated from who they voted for (see Florida, where the proposition lost at 57% support when the state voted roughly the same percentage for ol Don).

Harris had a potential base on the progressive left, but the DNC insisted on tweaking her campaign to try to win over right-moderates. That doesn't work anymore, precisely for the "sticking fingers in ears" attitude you mentioned from right-wing voters. It's asinine for the DNC to continue to try and appeal to them, when the median Republican voter thinks Democrats are agents of a satanic agenda. Regardless, the message the DNC seems to have gotten from Nov 5 was that they lost this election because they failed to move to the right hard enough. The ratchet effect continues.

As a side note, I know several trumpets who would've voted for Sanders in 2016 were he the Democratic nominee, and would've voted for Walz even this election were he the main guy on the 2024 presidential ticket. Such people are not very coherent ideologically, they just want someone in who has big ideas.

Unfortunately, it's just not enough to be "not the other guy", even if the other guy is a convicted felon, rapist, and just all-around a downright awful human being.

edit: grammar and wording in a couple spots