Why do we still don't know exact birth & date year of the Buddha?
Knowing the precise date of the Buddha's birth is not very useful knowledge within the Buddha's teaching, as it is presented in the Pali Suttas. How will knowing this precise birthday help any student transcend their own suffering? Only the most worldly and materialistic forces within any tradition would care about such superficial things.

The Pali Suttas(and arguably the Sanskrit Sutras) are the closest to what the Buddha actually taught. All traditions derive their truth from the Suttas/Sutras and other truths that might or might not be so compatible with the Buddha's actual original teaching. These additions were made for reasons of culture, politics and practicality IMHO.

If one wants to know what was likely the truth that the Buddha taught then one might stick to the core teachings that the Buddha repeated over and over again in the Suttas/Sutras. 

It seems to me that these core teachings fit within themselves with great precision. Their potential is so awe inspiring to me that I wonder what, in 2500 years, has been stopping so much of humanity from embracing the Buddha's teaching or any teaching like the Buddha's teaching.-Metta :)