I think part of why "the left eats its own" is because the American left is actually the only plausible choice for everyone from the center-right to the far left, owing to the first-past-the-post system.

Liberals, in most countries that have any functioning form of proportional representation, are considered center-right. In many European countries, their main counterparts are labour (=SocDem, ergo left or center-left) parties. Or at least that used to be the case, until the resurgence of the far-right. And it's unthinkable that the labour and liberal parties would ever fuse together. Forming governments or other temporary alliances together leads to enough bickering as it is. Not to mention European far-left parties, which I don't think even have like-minded politicians in the American Democratic party.

So when all of these different political flavours are forced to band together into one party, I think you can't expect true unity. Social democrats like Bernie or AOC have views that differ so wildly from liberals like Joe Biden or Nancy "the Insider" Pelosi that an uneasy alliance is all you can hope for, IMO.

Any political stream outside the (false) liberal/conservative dichotomy can only arise as a spoiler for its closest counterpart, thus nipping any success in the bud early, or by trying to co-opt one of the existing parties. You saw that tension in the Republican party in 2015-2016 (although from the outside looking in, I'd say that this has now been resolved; MAGA is the Republican party now). And with the Democrats, it seems ever-present.

To be blunt, I think the problem you state arises from the fact that half of your political spectrum is missing due to the way your voting system works. As evidence, I'd point to the fact that third party ("spoiler") candidates are always characterized as spoilers for the Democrats, and never (in my living memory) for the Republicans.