@DerTapfere@swiss.social @casandro@f-ckendehoelle.de @bsm@swiss.social not every US citizen is required to have a SSN.
What it comes down to is this: there is no legal requirement for all people to register their residency or even existence in the US. There is no dataset today with enough information to determine the eligibility to vote for everyone. A lot of it is related to history, including how Europeans settled here, how the US became independent, what people came here and why and who lived here before Europeans conquered the land.