Because there are in fact girls as young as 14 that are pretty good looking. There's quite a few in my school district.
I mean hell, most put on a lot of make up and tight pants, so they'll look they're 17-18 years old yet you'd never know. Until you find out they're only a freshman/sophomore or you ask/they tell you the age.
I don't find it weird or sick at all that males find young girls attractive. If the girl was really young like a child, then that would be one thing. But a girl age 13 (assuming she's developed...) to 17? No, because like I said, there are some girls that look older than they are, and are pretty good looking.
There's a term for girls/guys (usually girls I guess) that are under the age of consent. It's called Jailbait. I turned 18 in March, yet that doesn't mean I don't find girls under the age of 18 attractive. There's plenty of girls age 14-17 that I find either really cute, hot, beautiful, very sexy, etc.