A Social Security Number is not an ID expressly as a matter of law, because it can’t be legally. The many loopholes the Federal government tried to use to backdoor a national ID were shutdown by the Supreme Court repeatedly. The US can only have a mandatory national ID system if the individual States, in aggregate, decide to create one. Thus far, they have shown no interest. Real ID is not a unified ID because the Federal government cannot compel it.
As with most persistent problems, the “obvious” solutions are not being ignored because no one has cared or no one has tried but because there are fundamental technical reasons they don’t work.