This is a reason why many amerikan Internet communists believe this.

Lots of Internet communists lack discipline/patience etc of being dedicated revolutionaries. They want communists somewhere to be successful but They don't want to do the extremely difficult task of long term revolutionary organizing, which can take decades to bear fruit, if it ever does. There's a high chance you will spend time in exile or in prison. And there is never a guarantee of success. Therefore there is a tendency among Amerikan comrades to emphasis the material conditions which make revolutionary organizing difficult in the usa as the reason the Amerikkkan left is inherently useless, and therefore they can justify there spectator-like relationship to politics. They can watch from the side lines and root for the socialists/anti imperialists instead of taking and difficult and dangerous agentic role in politics. (Notably comrades from other countries often do this as well, it's not solely an American problem)

It is true that the material conditions in the usa do not currently seem conducive to revolution, and that the state of the US revolutionary left is severely underdeveloped to put it politely. However to conclude that the US proletariat/left is entirely useless and should not even try is missing two points. Firstly, we can't actually tell where revolutions will succeed, least of all decades in advance. Many Bolshevik leaders didn't think they would live to see a socialist revolution in Russia. If they had given up on the organizing project, concluding that the material conditions were better in Germany or France, there would not have been a Bolshevik revolution in Russia. Secondly, by living in the usa you have many opportunities to disrupt the imperialist war machine, in acts of material solidarity with global anti imperialist forces. Whether or not your organizing actually results in a socialist transformation of the usa (which I hope it does) the opportunity exists to do some serious good.