In Seth Godin's usual, preternaturally succinct style, in his post "How'd It Work Out", he writes:
Doctors and consultants and builders are often hesitant to ask about how something worked long after the work is done. It feels like nothing but a chance to hear a complaint.
Professors are perhaps the worst at this. So many academics have invested so much ego in how they teach what they teach, the thought of finding out years later that their underlying assumptions are wrong is terrifying.
Plus, the typical course evaluation method occurs formally at one point: at the end of the semester in which a subject is taught. But if what you are teaching is truly resonant, its effects will likely not manifest until many years down the line.
Indeed, assessing at that level requires more resources than deans are willing to commit—they'd rather staff the whole place with adjuncts, and what they want to hear from alumni is how much they are going to donate. But that's to be expected.
We as professors have to have the courage to accept that we may be fundamentally wrong in our approaches. Academics as a whole are not very good at that. We may have excellent research methods when it comes to our areas of expertise, but when it comes to curriculum design and pedagogy all that seems to fall away. At every institution I've worked, when faced with modifying curriculum, I usually find myself faced with arguing against mountains of unfounded anecdotes and isolated data points.
What's wrong with being wrong?