Honestly, I can't say for certain why I am the way I am. Do you know why you aren't attracted to children? I could list some of the things I find attractive about girls that I don't find in women, but that wouldn't explain why I find those things attractive.
I can say for certain it isn't for the reason you started the thread with. I am an adult (physically), and I feel like an adult (emotionally), and I think like an adult (mentally). And yet I find myself drawn to girls that aren't even 1/3 my age.
It may not even be possible to understand without a deeper understanding of psychology.