But yesallmen/believeallwomen DID, as you say, come with widely available explanation and purpose.
…and you don’t think that choice of language played a part in the reaction to things like when Jimmy Bennett accused Asia Argento of statutory rape and she paid him off to shut him up? I mean, she more or less literally did what #metoo was accusing powerful men of doing, had done it recently at that point, and didn’t get half the shit most #metoo targets got as a consequence. Or that aide that accused a CA legislator of being inappropriate with him, so she investigated herself and found no wrongdoing on her part.
And even the people who claim to be all about #metoo and #believewomen get…picky about that when a woman accuses the wrong person - see Tara Reade’s accusation against Biden.