I guess its because the first 2 weeks you aren't actually pregnant and they say women normally conceive around 2 weeks after they period. I'm also wondering the same thing but I'm open to another explanation.
There is a historic reason. I've posted this before but maybe you didn't see it when you did a search on the topic. It has to do with how things were determined back before there were ultrasounds.
If a doctor, nurse, or any medical person says you're a number of weeks "pregnant", this doesn't count back to conception but to two weeks before conception, because that's when the official medical pregnancy time period (the gestational age count, or the GA) begins. All medical counts of the pregnancy time period begin with the first day of bleeding of the woman's last period before getting pregnant. (Or, if her cycles are irregular the ultrasound measures the baby and checks various markers, and assigns a day to begin the count that is two weeks earlier than the ultrasound suggests the baby was conceived. ) If a doctor says, "Congratulations, you are 6 weeks pregnant!" he or she means about 4 weeks since conception.
The count is done this way because the period is a big, obvious signal, not because the doctor thinks you are pregnant when you are on day 1 of your cycle. (The doctor knows you were not pregnant then -- you were having a period!) But back before ultrasounds could peer into the uterus and measure the baby, all a doctor had to go on as a place to start the count was to ask when the woman's last period came. Conception is hidden, but you sure as heck can see a period, and many or most women know when it came. The entire medical establishment uses this kind of count even in this modern age of ultrasounds. That is why pregnancy, which takes 38 weeks from conception to full-term birth, is counted as being 40 weeks long by doctors, textbooks, nurses, ultrasounds, everything. It's a common language and well known in the medical world, and continues to be used nowaways, to keep everything calibrated with the textbooks and the way doctors are taught. This leads to the rather peculiar phenomenon that the first two weeks of the GA count, the woman isn't actually pregnant yet.
Sometimes a woman's GA count from an ultrasound doesn't align perfectly with when the first day of her last period really was. This is pretty common if the woman's cycles are not like clockwork exactly every 28 days no matter what. Basically, a lot of women don't ovulate precisely two weeks after the first day of their last period. If a woman has an ultrasound that gives her a different starting date than the real date of her last period, as long as the ultrasound was early enough, it is more trustworthy than just counting from the period. The ultrasound sees and measures the actual embryo. If a woman gets an ultrasound, she should use the number based on the ultrasound.