I knew there was some story behind it (thank you rock rose) I have always just been Curious! :)
Because this went back generations when it took months to determine if a woman was pregnant.
The best "hard fact" they had was the first day of the last menstrual period, and after that all they had to go by was a doctor feeling around on the uterus to determine she was pregnant. Then there was the stage where they had the rabbit test, injecting urine from the woman into the rabbit. If the rabbit's ovaries swelled, the woman was pregnant. (Poor rabbits).
Anyway, it just dates back to a medical time period hundreds of years ago when they didn't have the technology to date conception as they do now.
Because you can go two weeks over 40 weeks or two weeks under 40 week
They can't know for sure when you conceived, but you should definitely know when you had your last periods.
I'm not sure but it just makes it easier if everyone counts it the same :p