One of the things you're told when you're part of a large consulting firm is that 'no matter what the problem, there's someone in our organisation who knows the answer.'
And that's tremendously reassuring, because if a client corners you with a question on a topic that you've got no clue on, (which only happens close to 95% of the time), then you can fall back on the line 'well, we have an expertise in this domain, and I'll be happy to introduce you'.
It is an article of faith that this is true.
As Strategy Man alludes to, the real problems come later in a) finding the expert in the haystack of consulting bodies, and then b) getting them remotely interested in helping you. Neither of these constraints are insignificant hurdles to overcome.
Consulting is as much about selling internally as it is to clients.
So true, Consulting is art at times