I’m guessing that when Social Security started being paid out in 1937 that many US citizens were born around 1875 and also did not have a birth certificate. So

I’m guessing that when Social Security started being paid out in 1937 that many US citizens were born around 1875 and also did not have a birth certificate. So when the system was computerized in the ‘70’s there was no way to correctly input data so they simply continued that method.