the difference is all of those countries had established left-wing parties, cultures, and institutions

America has absolutely none of those institutions right now (not even as a hollowed-out shell, since anything remotely resembling the New Deal has been purged a very long time ago from the Democrats) and basically has to rebuild them from scratch; there is no electoral or organizational capacity that is stable or extant; you have to build all of those institutions to actually be able to do literally anything, and sorry, you’re going to need to appeal to deeply propagandized people who probably have no fucking idea what a labour union actually is or what socialism means

this isn’t entryism because you’re not really expecting to change the party, it’s just impossible to be elected as a third party in America with how ballot access works

you can’t just look at Europe and apply it to America