There's probably nobody more frustrated by these sudden book shortages than the books' authors themselves. Being out of stock during the sweet spot of this year too often means the book sale will never be made.
I'm working on a microbindery, which involves ultracheap one-off book runs in tiny distributed shops around the country, the operative idea being that instead of waiting six weeks for the next containerload of today's bestseller/tomorrow's pulp, whatever's out of stock can simply be knocked out in three hours and either picked up or mailed a short distance.
Making a book (a real book) takes more than your $39 Deskjet, but it's not hard, or all that capital- or even labor-intensive.
A single command line argument could convert a PDF to a perfect-bound novel or textbook, indistinguishable from the factories that produce books by the containerload but in massive MOQ's and haphazard sales and proof processes.
Even if it costs $15 instead of $12 to print bestseller copy number n+1, that's still plenty of money left on the table if you're at the mercy of a massive 20th-century pulp mill in a different time zone, who can't be bothered with your last minute business.