Isn't the obvious reason that search is perhaps the best service to sell ads for? And Apple has the benefit that the cult would be extatic to be walled off even tighter.
I don't think Apple search would provide any tangible benefit for Apple users, nor do I hear any regular iPhone users express any desire to have Apple search, they just use the default. It might benefit Apple though, but why spend money on developing search when you can just use Google and get 36% of the revenue.
The whole wanting a 100% Apple ecosystem isn't something I think many a clamoring, except perhaps the few insane hardcore Apple fans. Also currently the alternative is pretty much locking yourself into a Google ecosystem and that's worse.
Apple of course does what benefits Apple, regardless of whether it benefits the user. 100% is 64 percentage points more than 36%. And due to the lock-in Apple could do away with significantly worse search than Google.
I agree that the heavy and intentional un-interoperability of Apple products is not a benefit for the users (although some hardcore claim it is). E.g. Apple clearly benefits from preventing sideloading (to force the App store rake) or alternative browser engines, but hard to see this having at least net benefit for the user.
The whole wanting a 100% Apple ecosystem isn't something I think many a clamoring, except perhaps the few insane hardcore Apple fans. Also currently the alternative is pretty much locking yourself into a Google ecosystem and that's worse.