Part of the issue is the push by many left-wing voters to get actually progressive politics on the table after years of alternating between regressives and complacent centrists* that prefer making small concessions to the right over big steps to the left. They don't want another presidency marked by lukewarm promises kept poorly. They're tallying up all the ways in which Harris still isn't as good as she ough to be.
For Trumpers, he is good enough. He is everything they want: A public role model enabling them to be an absolutely shameless asshat.
The complexity arises when people advocate voting for a third party instead. By and large, no third party has the traction to beat the Republicans. You'd need to get the entire Dem voterbase and then some. If that fails, you've split the non-Rep voterbase and the enabling asshat gets the plurality. On the other hand, there's a risk that leaning too far left in the attempt to keep the progressive voters may lose the centrist* voters, which is a gamble whether that will end up a net positive. Harris has a tough job: walking a political tightrope, particularly if it's consistently being tugged at by people.
And there are good reasons to tug on that rope. You'll find some in these comments: Settling for "Good enough" doesn't help getting actual change. For the ultra-rich, on the other hand, progressive policies are a detriment, so they'll want to tug it the other way. The left doesn't want to cede ground and keeps pulling. The centrists* that don't like Trump but also fear dramatic change pull her to the other side again. The "centrists"** pull just to see her fall.
And that's exciting! That's an actual conflict of ideologies! That's her having to work for her voters' approval! You'll see the complaints flying left and right, see her try to keep an ever-shifting balance, see drama and tension! People love drama and tension. Corporate media loves drama and tension because it gets attention, clicks, revenue, all that. "Assholes still support Asshole" just isn't as interesting as " criticises Kamala for , calls her ".
Also, splitting the Dem voterbase serves the corporate executives and shareholders that want the right-wing tax breaks and erosion of worker protections because it makes them even richer. That's probably not a coincidence.
*Centrist as in "I don't want things to radically change", not as in "I think both parties are equally bad, so I'll sow dissent in the Dem voterbase, pretend that I'm not helping Trump with that and get to feel superior to both".
** The latter group of the above footnote. It doesn't really matter whether they're intentional agents of disunity or idealists that care more about voting with their heart than the actual outcome. The result is the same: At best, they've achieved nothing. At worst, they've contributed to Trump's victory.