By the time your essay-writing ability has gotten you into college, you’ve internalized a few lessons. One is that a longer paper will probably get a better grade. You read academic writing, which is full of passive voice and jargon, and learn to imitate it to sound smart.
Too simple, but he's on to something with the negative feedback loop of all those unedited Huffington Post blogs, etc.
The actual reason is that we no longer read (real) books as a society. I learned to write by reading. It's easy, basically a shortcut. Learning to write by taking a writing class is self-defeating and torturous. It's like learning to speak by thinking of definitions and sounds word-by-word instead of going off internal deep knowledge.