I am sure by now it has been explained by others. But basically - an ID document is like a bearer token that does not need to call a central authority every time it is verified. I am sure there are cases where it is, but a digital token that is linked to location
every time it is verified is a quite different thing. Currently in the UK the law states that ultimately only a court can force you to identify yourself - by which time hopefully the purpose for which identification is being done is quite a serious and valid one. Making it cheaper to track people is not exactly a goal worth pursuing in my (not so humble) opinion.
To add to this - there is very rarely in my mind a need for someone to actually identify themselves - there are plenty of examples where it's useful for *audit* purposes to have a record, or to have a role-based credential to be able to do a thing, but *identity*?