The time of pregnancy is calculated from the first day of your last menstrual period (LMP), which is typically two weeks before conception actually occurs. This means the first two weeks of the 40-week count are before fertilization, during which your body prepares for ovulation and a new cycle begins
Of course it does.
Do you want to tell so many people the exact time when you banged her/got banged by him?
Well, maybe you want. Maybe you would even say that you can’t tell the day exactly because you did it every night for 3 weeks in a row. But not everybody can, or wants to, talk about these things, for all kinds of reasons.
Therefore a general way is neccessary for determining a date that serves as the begin date, for medical and legal purposes. And this one has worked best so far.