Why is the western left so anti-communist?

LoL. Certainly possible it’s multiple people coordinating, but I think it’s the same lonely loser pretending to be a group of assholes. Same speech/text patterns, and you can see weird timing on comments that look like someone switching between accounts.

Sanders is a social democrat, ie capitalist but a supporter of enlargening safety nets, not a socialist. Mamdani claims to be more of a reformist socialist than a social democrat, but that remains to be seen. You are correct, support for socialism and communism is rising in the US, but the way you frame it is wrong.

"Authoritarianism" is a buzzword. It doesn't really mean anything when you hide from class analysis. Socialist countries wield authority against the capitalist class, landlords, fascists, etc, while capitalist countries wield authority against the working classes. There isn't some arbitrary scale of "libertarianism to communism" where the more radical you get the more the state acts. The citizens of the US Empire aren't especially independent, and the system isn't lopsided due to electoral habits, but due to systemic structures designed to perpetuate capitalism and imperialism.

The US is authoritarian against the working classes, and that's by design. The opposition to socialism and communism from older generations and liberals is more a product of the Red Scare and cultural hegemony of the bourgeoisie than anything else, alongside class interests benefiting from imperialism.

"Authoritarianism" is not a word that describes any real distinction, it's a thought-terminating cliche that boils down to "any use of authority the speaker doesn't like", similar to how "terrorism" just means "any use of violence the state doesn't approve of". In practice, there is no such thing as an non-"authoritarian" government. It's an oxymoron. Every single government on earth maintains a monopoly on violence and enforces laws with that monopoly. It's doubly important to use that authority to protect a revolution, because no capitalist order will allow itself to just be voted away without escalating into full-spectrum war, as we've seen for over a hundred years, and will require a functioning state to combat. From Engel's On Aurhority, 1872:

All Socialists are agreed that the political state, and with it political authority, will disappear as a result of the coming social revolution, that is, that public functions will lose their political character and will be transformed into the simple administrative functions of watching over the true interests of society. But the anti-authoritarians demand that the political state be abolished at one stroke, even before the social conditions that gave birth to it have been destroyed. They demand that the first act of the social revolution shall be the abolition of authority. Have these gentlemen ever seen a revolution? A revolution is certainly the most authoritarian thing there is; it is the act whereby one part of the population imposes its will upon the other part by means of rifles, bayonets and cannon — authoritarian means, if such there be at all; and if the victorious party does not want to have fought in vain, it must maintain this rule by means of the terror which its arms inspire in the reactionists. Would the Paris Commune have lasted a single day if it had not made use of this authority of the armed people against the bourgeois? Should we not, on the contrary, reproach it for not having used it freely enough?

For something.more modern, here's a sick ass 2 minute Parenti beat that goes over the same thing

Downvote isn't mine btw, I save that for people who are being hostile