Why do we still don't know exact birth & date year of the Buddha?

I just want to know:
1)Why do we still don't know exact birth & date year of the Buddha?
2)What are the factors that is limiting in knowing Buddha's birth & death year?
3)Why many people/countries/traditions contradict? Many countries following different traditions, follow different Buddhist calendar year i.e. years since Buddha left.
4)Even the recent Nat Geo Excavations reveal he might be born in 6th Century B.C. A 2013 news which still doesn't official claim nor does it leave a convincing conclusion till date, on whether their findings are indeed perfect or not. Nor their findings seems to be accepted if its true.
5)Does the pali text gives any reference to the same?