I have a theory. Antipsychotic medications just need to make the person more "manageable" to be considered effective. It doesn't need to eliminate the psychosis directly, it could just make the person more sedated, and it would appear to be "working" properly.
but also, if you've had a conversation with someone in active psychosis, vs. on the right dose of medication, there is a really incredible distinction in the clarity of their thinking and speaking -- night and day sometimes. so the drugs do work at genuinely treating some of the problems. and good psychiatrists do look carefully for this kind of improvement.