If the work of some teachers could be transformed into its equivalent in carpentry, the result would fill an assembly hall with distressed furniture: Uneven bookshelves, stools with wobbly legs; marred coffee tables, and desks that collapse at the slightest touch of an elbow. Nails would protrude, surfaces would be rough, and the use of a level would reveal that a level had never been used.
Think back on your own education and tabulate how many history teachers made the topic boring beyond belief, how many math teachers turned their classrooms into a reign of terror, and how many science teachers could not stir the imagination if you held a gun on them. We won't even go near the foreign language department.
At some point, those individuals either forgot their craft or decided it wasn't worth learning or improving. They became classroom monitors and not teachers.
Consider how similar transformations occur in other professions. Someone gives up on the primary job and starts to do a secondary one, and on the day that happens, the shelves begin to slant and the legs begin to wobble.