I've seen scenarios where UUIDs were used intentionally for this, to prevent users who get a full export of table data to reference the UUID. If it's just a sequence of small integers they might them to more permanent and intrinsically tied to the data, whereas UUIDs seem more like computing artifacts.

Yeah, I know, this could easily fixed by not friggin' showing surrogate keys to those users in the first place, but, well, data integration is ugly business.