I’ve said this in another comment, so likewise I’ll preface that I’m not from the US so I use liberal as in classical liberalism, but under a materialist interpretation that I’m prone to agree with, liberalism is a bourgeois conservative ideology, thus a liberal is inherently in favor of the status quo and capitalism. So really, it’s not about perfection, but rather an irreconcilable difference, or a contradiction, if you will. This is not to say that progressive liberals/social democrats can’t enact meaningful material reforms, but usually not without confounding factors.

As for excusing Stalin and dismissing the opposition from progressives against the real threat of the far right, can’t say I’m fond of the tribalism so common, but not exclusively, with MLs either.

I’m not from the US, so I might be misinterpreting something, but I imagine that it stems from a sentiment that liberalism is inherently in favor of the status quo and capitalism, which makes sense from a materialist perspective, and thus liberalism would be a bourgeois conservative ideology, under that interpretation. A view that I also tend to share, mind you.