Apple can buy the same search index access DDG has (and probably would have to re-negotiate DDGs existing contracts when taking them over) if it wants to get into search. Although DDG has some additions on top of that, if it'd be cheap enough those could be a reasonable deal for Apple.

I'm not sure Apple wants to get into the mess that offering a major search engine is.

Google is paying them quite well for the opportunity to be the search engine on Apple devices.