Because it's not a subject that is EVER discussed in a sexual or medical capacity so when it happens the first time we think we have actually lost control of our bladders and get embarrassed and hope the guy doesn't think we just peed on him.
Then we leave the situation contemplating WTF did I just pee on him? And we are so confused we don't bring it up with our friends to resolve. I have never talked to my girlfriend's about this and they have never talked to me about it.
Eventually you hear about it second hand, in a magazine, on a website, erotica, and you think "hey, okay, maybe that is a normal thing!?" But you still don't have legitimate confirmation.
It would be different if young women were told "sometimes, when you are very sexually aroused, you may experience a rush of liquid .... Etc, etc, etc". Then, at least, we could associate it with a positive event.... "Oh, wow, he really does it for me"
But nope.
Make sense?