Why are people so sure that just because a woman has her period then she can't possibly be pregnant?

I know that doctors and scientists have stated that this is a fact, but I know of alot of women (myself included) that have had regular periods during pregnancy,(not implantation bleeding because they are completely different and do not last for months) and went on to have healthy babies, not miscarriages like ppl are claiming. I just feel that if you never experienced it then you shouldn't tell the women that are TTC, or are having other signs of pregnancy and thinking that they may be pregnant that there is noway they can be pregnant just because they had a period. Because that isn't a fact. My suggestions to the women that aren't sure of weather or not they are pregnant is to take a test and/or have their doctor check them, because that is the only way to be sure.