@platypus @futurebird @phette23 I'd say it isn't that rare. But I guess the publishers that really engage in that practice perhaps don't generally publish books that are of interest for library patrons. Because they churn our pretty generic stuff.
Also publishers of romance novels don't necessarily care for ISBNs
@futurebird @phette23 ISBNs get assigned in blocks to publishers by the national authority. Discount publishers might retire a title and reuse its ISBN so they won't have to buy another block to publish additional titles. You cannot order the older title anyway, they figure, so the ISBN points at one title in print.
And then there's supermarkets and retailers that don't even bother to assign ISBNs and just use EANs. They're not in the book trade: they're selling *stuff*