Why is it so difficult or usually impossible to find out someone's
e-mail address using the multitude of existing 'directories'. I have
never been successful yet.
Why cannot an individual opt in to one comprehensive list giving name
and email addresses for home/work and also other family members'
details?
Am I missing something here?

Very few people choose to opt into an email directory. Most people
probably don't even know that they exist.

If you really want even more spam than you're getting at the moment,
by all means opt into an email directory!

I think charliebabbage has pinpointed the main reason why folks don't
want to be listed in email directories. Most of us have more than
enough spam already. A public listing in an email directory is an
invitation to spammers.

The comments above are valid, and I agree with them.

I'd also like to add the perspective of myself an an email
administrator.  I control the email system for the three companies in
our group, plus my own email addresses within my own domain.  Would
you see it as my "duty" to update some central email directory with my
details?

In order for you to find *me* in this, I would not only have to say
that I am "Glen Monks <exmaple-email@archives.ac>" but also give
partial or full postal address details so you could at least know that
I'm the Glen Monks from "X Example Street, Bradwell Common, Milton
Keynes, UK" and/or give my date of birth so you could at least find a
29 year old Glen Monks that you mgiht have known at university buy
don't have an address for.

Who would I be giving this information to?  I certianly woudn't trust
the email directories on the Internet such as "where are you now.dom"
or somesuch - they're only in it to collect details so they can sell
them.

Perhaps you picture the government to hold this almight database?  I'm
in the UK, chances are that you're in the USA and I sure as hell don't
trust your government with my details.

I said I administer the emial accounts for the staff where I work. 
WOuld I ahve to gather and publish this name, address and date of
birth information on their behalf, or would it be the responsibility
of each individual?

If people want to be found, you can generally find them.  Type "Glen
Monks" into Google and you'll certianly find me - I've even got an
AdWord for my own name.  :-)  You'll also find a drugs researcher in
London and a (deceased) firefighter.  If people don't want to make an
effort to be found, then directories won't help.

To finish on a brighter note, in my opinion it's not going to be long
before the phone companies start adding email addresses to their
directories.  I bet in San Francisco, they already do.  :-)  The
problem of spam is the biggest hurdle, but there is increasing
discussion of the "electronic postage stamp".  If someone is prepared
to pend a few pennies on contacting me, then I'm prepared to read
their email.