Memes

People will casually remark "that's just they way they're using the word, no point in arguing" and then never stop to ask why and to what end.

The reason US politics operates along that dimension is explicitly because of the way their electoral politics work. It's not simply a matter of it being the common usage, it's also a core part of what capital L leftists are critiquing when they say 'liberals are not left wing'. Democrats are dealing in exactly the ideological framework that is the target of leftist opposition, it isn't sufficient to lump everyone on the democratic side into a single category.