@gimel: It's... complicated.
The US maintains two major ID programs: driver's licenses, and Social Security numbers. Neither of these was meant to be a universal ID, which shows, but they were widely adopted because it's convenient.
In general, if you're a US citizen, a legal migrant, or have a visa, you're given a Social Security number and card (there are a few exceptions, like the Amish). Since everyone has a unique number that's tracked and verified by the federal government, this is used for things like bank accounts, jobs, tax returns, etc. However, the card doesn't have a photo or date of birth, so it's not really a primary ID in that sense. It can't prove that the person holding it is the card owner. In fact, cards printed before about 1970 expressly stated that they were not for identification purposes.
On the other hand, driver's licenses (and their lesser-known cousins, state IDs) are the de facto photo ID. However, for historical reasons they're run by the states rather than the feds, which means it's an unholy mess trying to verify them, and it's a hassle if you move between states. Also, since it was originally a specific license, it has a registration process and fees (like a hunting license or gun license), so it's less than universal.
Basically, like most of the weirdest things in the US, our ID system grew organically over the last century into something that almost, but doesn't quite work for most people. And with the current political climate, nobody trusts anyone to monkey around with the system and make it work, for fear that they'll turn it to their own ends.
