It's not true that those who can't do, teach (some of the best hackers I
know are professors), but it is true that there are a lot of things that
those who teach can't do. Research imposes constraining caste restrictions.
In any academic field there are topics that are ok to work on and others
that aren't. Unfortunately the distinction between acceptable and forbidden
topics is usually based on how intellectual the work sounds when described
in research papers, rather than how important it is for getting good
results. The extreme case is probably literature; people studying literature
rarely say anything that would be of the slightest use to those producing
it.

Though the situation is better in the sciences, the overlap between the kind
of work you're allowed to do and the kind of work that yields good languages
is distressingly small. (Olin Shivers has grumbled eloquently about this.)
For example, types seem to be an inexhaustible source of research papers,
despite the fact that static typing seems to preclude true macros-- without
which, in my opinion, no language is worth using.

Paul Graham
"The Hundred-Year Language" - http://www.paulgraham.com/hundred.html