It is (or was) difficult to process a phone book at the rate of 100,000 entries in a few minutes or seconds. Electronic data gives you that capability.

The information in a phone book is now the backing and access data, at least in some cases, to other online systems and services. "Who are you?" is the most expensive question in information technology. No matter how you get it wrong, you're fucked.

There are a great many systems which now can use the information in a phone book, or which starting from a phone book you can access, to commit fraud (it's called "identity theft", but it's still just plain old impersonation fraud). Unfortunately, this becomes, in many jurisdictions, the victim's problem.

It's not so much that phone books have changed (though that too), but the world around them certainly has.

Identity theft is possible for the sole reason that privacy advocates consistently defeat proposals to build any system of identification/authentication less broken than forgeable drivers licenses and shared-secret SSNs. People actually get angry when we make it harder to forge IDs! Propose any sort of PKI or .gov OAuth provider or something, and you're basically Big Brother himself. It's like the ability to commit identity theft is considered a right.

Trying to authenticate people by their knowledge of data that's in or easily deduced from the public record (or "shared secrets" shared with every entity they've ever done business with) is an objectively wrong security design and we need to stop using it. Making those public records harder to access is an absurdly circuitous fix to something other than the real problem.