Why do some people call music melodic which isn't?

its a colloquial use to begin with. something can't really be more or less melodic. it either has a melody or it doesn't. that being said, we know what they mean...

the ironic thing about your example is that punk/garage tends to reuse golden age melodies, specifically late 50s early 60s rock and roll, rhythm and blues and surf. which goes back to the "we know what they mean" thing, the melody might be the exact same, the arrangement might be similar, but have a punk singer instead of a note-hitting-centric singer, have an overdriven guitar instead of a reverb guitar, and people perceive it as less melodic.

#1- most people are not musicians, so apart from having the right to a personal taste, their technical opinions don't matter

#2- rap is no-melody, no-harmony = not music

#3- all music is melodic, including "least" melodic genres with screams and growls.

Because people are looking at it in loose terms. Not the textbook definition of it. Interpretation is a huge part of it. You look at ot differently. Nothing wrong with that.