The point is that you can’t assume any particular ability available to the party when designing monsters for a table-agnostic design space like the Monster Manual.
The inflation of save DCs on monsters in excess of proficient saves at high stats is not justified by the existence of Paladins.
And whilst I don’t think they did design it that way for that reason (I think they did it because they didn’t think very hard at all about how they’d designed saves) people have brought it up.