There are many ways a book can be rare. First editions, signed copies, and books with little demand. Can’t fire up the printers and make those again.
In the case of little demand, I have a book “Two thousand leagues under the seas” not the more common “Twenty thousand leagues under the sea”. It’s rare in that it’s very difficult to find more literal translation than the version we are accustomed with. I suspect search engines simply think I’ve made a typo. Either way, why print it if there is little demand or can’t find the book?