A question - Girls' bikes and boys' bikes - Why the difference?

I don't bike much, but I do enjoy wowing people with useless
facts, so here recently when someone asked me why boys' bikes have that
horizontal bar between the seat and the handlebar, whereas girls' bikes
have a curved one, I was stumped. I tried searching on Google, but I
couldn't find any answer to this, so I figured I'd get the answer straight
from the horses mouth. Someone care to enlighten me?