The difference is that not every state requires an SSN to obtain a driver’s license, and voter registration is granted provided you’re a citizen. An SSN is typically provided at birth or with granting of citizenship, but an individual or their parents have to apply for one (I think). Also too, resident noncitizens that work in the US are supposed to get an SSN for IRS reporting purposes. This is why SSNs aren’t typically considered a citizenship document.