Apple has clearly defined that the customer for the "default search engine selection" option is Google and not the users. There is obviously some mediation by regulators in Europe and elsewhere (hence Google's major competitors being included on the list), but the customer is Google. I think that from that fact flows two further inferences: A) the customer might not want to write down everything, or even communicate in any non-deniable way, all of their preferences (due to those regulators), but can presume that Apple understands their preferences B) The customer is happy to the tune of 20 billion dollars with the current set-up.
Something that doesn’t make a ton of sense to me in this theory is that, despite it being hard to add a new engine (which is not great), it is easy to switch away from Google on iOS. And the big search engines are in their pre-populated list. So it seems Google and Apple have engaged in a conspiracy to keep people from switching… just to the niche engines? That doesn’t make a ton of sense, right? Google is probably not more scared of Kagi than Bing.
My first guess is that Apple lived through the era of confused non-technical people adding a bunch of scam search engines and didn’t want a repeat of that.
Google certainly should be more scared of Kagi than Bing. Bing is a known quantity that's not going to suddenly shoot ahead of them. A smaller, newer competitor might. Just like how it wasn't Lycos or Jeeves that dethroned Yahoo, it was the two guys in a garage.