Because really it's not going to happen.
Offered to sell my buggy to someone today and loads of people said "oh you shouldn't, because as soon as you do you'll get pregnant". Er after three years I don't think so. and really, it's ok, I'm beyond the point of being sad about it any more. .
But why is it that just because everyone seems to know someone who gave up trying and was suddenly pregnant they think it happens to everyone?