Because no other search engine provides nearly as smooth an experience as Google.
Coincidentally, nothing is stopping privacy-conscious users to use, for example, DuckDuckGo. It's right there in the options on both macOS and Safari.
Finally, it's pretty absurd to say that Tim Cook/Apple don't believe in privacy. Out of all major consumer software/hardware manufacturers, Apple easily takes the most privacy-conscious stance of them all - which isn't saying much until you see the extent that they've gone to protect user privacy - ranging from the Secure Enclave and the T2 chip to fingerprinting and tracking prevention in Safari 12.
If he was being honest, Cook would have said: "Privacy to us is a comparative advantage, because we can't make world class web products."
You can see the difference between these two statements in action in cases where Apple absolutely needs government approval and market share (China) or a good product (search engine / Google), they immediately ditch this "human right".
That comparative advantage has also made them the most valuable company in the world so your normative lens may be a bit biased — it seems like one could easily say googles comparative disadvantage is their business model precludes then from offering lucrative high margin privacy-protecting products.
The point about maps feels rather non germane as apple does have its own maps product.
Not sure this is true. Just yesterday I had to explain to an average user how ddg was answering a query they couldn't get answered in google. This was a really basic thing (finding the NYC office address of a company we were working with). For some reason Google just wouldn't return the result whereas it was top hit on ddg.
My experience is the exact opposite. DDG works well for most things, especially programming, but when I want to find the address of a local business Google is much better, probably because they use my location.