At least some of them must use it but for general correspondence only.
After all, Dr. Grossan has frequented this newsgroup; Dr. Wellington
Tichenor takes general questions via email; etc.

If you mean why they don't email patient-related information, it's
likely because of the risks to security and privacy. Either they (and
their patients) don't know about encrypted email or they don't trust it.

These days, there are simple (and free!) solutions, such as GNU Privacy
Guard.  GPG works with both Outlook Express and Mozilla-Thunderbird, and
it incorporates the latest encryption algorithms.  So even a patient of
modest means can send and receive encrypted email.

I don't think encrypted email is yet available for hand-helds like
Blackberrys and cell phones (though it could be someday).  So you still
wouldn't be able to receive your doctors' email that way.

A physician friend of mine explained that it was purely a liability
issue.

What if a patient, naively, sent an e-mail on Friday, at 3pm, saying
that he was feeling mighty ill.  What if the doctor's office was
already closed for the weekend.  What if the patient had something
serious, and died as a result.

Somebody would likely sue the doctor for "having knowledge about the
patient's symptoms" and "failing to act."

That SORT of situation, anyway.

Who knows ... but it was a plausible argument to me.