even leftists absolutely do not take fascists seriously enough.

A lot of this directly has to do with their own fascist tendencies, like the rampant ableism. Fascists are not some inhuman force incapable of thought and decision making. Fascists are not "stupid", they simply do not care about what we consider truth. Trying to reverse their hierarchies on them not only does not defeat them, it strengthens the hierarchies they use.

People see fascists as some inhuman force incapable of what they consider logic because they are afraid of the idea that they might use fascist strains of logic. This is exactly what promotes it happening though. The only way to actually fight fascist thought and propaganda in yourself is to acknowledge them as human. To acknowledge that it might be there in the first place.