For one reason, a few of the girls, though exactly how many I can't say, fell in love with Cloud, while identifying themselves as Aerith. So Aerith can't love Zack, so Zack has to be everything bad or not as good as Cloud. The same applies to Cloud not loving Tifa, because she's nowhere near as good as Aerith they are, natch.
It's an emotional attachment to an ideal they can't get away from.
Hilariously, this is much the same thing they rag on Tifa for and can't ever admit to Aerith doing, even though the reverse is closer to the truth.