Because they have to provide them for various reasons from time to time. I doubt that anyone besides the jurisdiction where you were born has an ORIGINAL birth certificate. They are all copies and they are certified to assure the person checking that they are accurate.
What I am thinking is that there might exist 2 forms. One is the hospital copy with the weight and height, doctor’s name and exact time of birth. As the mother and baby are released home, they might be given the copy of the hospital record which is the proper birth certificate.
At the same time, there might be another form that certifies that you were born then and there. I requested a form like this for my kids when they had to receive their passports. It’s a much shorter form, and it’s format may change. For example, if I request a copy of the BC in 1997, it might look one way. If I request a copy for the exact same child in 2007, the form might look different. So we need a comparable form from 1964. Does it make sense what I’m trying to say?
Oh, to add. My husband has his original birth ceriticate (from a European country). It’s a bright orange pieace of cardboard and it has the exact time of his birth (which I used to do his horoscope). But if he goes to the authorities to get the certified copy, it would be an A4 white piece of paper with less information. It would not have his time of birth, for example.