Because if you compliment they assume that you fancy them and it creates awkward situations. Also, i see no worth in complimenting someone on their looks anyways, not that i've ever met a man who I felt compelled to tell them they look good other than the ones that i know personally if they asked for my opinion. I don't see it as my (or anybodys) place to say anything regarding a persons appearance if they didn't ask for opinion.