Why a harem?

Why does so many anime have harem's in the is it a custom. Or something I'm not getting I've watched anime since I was seven years old and I'm still imberresed when I said I wanted a harem when I was a little boy to my mom she said we were Christians so it's not allowed.

Because it has to appeal to coomer fantasy. No coomers, massified manga/anime industry gets no buccs. And the only branch of Christianity that allows polygamy (but only for men), hence a harem is the Mormon branch.