But what would be their incentive to do so? Normally they launch products and make it exclusive to their devices so more people will buy iPhones, but that is difficult to do with a search engine. Otherwise they would have to get into the ad business like Google.
Apple is a publicly traded company, and every company needs to grow into new markets to make more revenue. And they also maintain their own browser Safari, even though on macOS they could just withdraw from market and leave the field to Chrome and Firefox. Even amongst macOS users Safari usage is very low and doesn't make Apple any money.
On the other hand you can see how Google is using its dominance in Search to push its browser and mobile OS - once you login to Google in Chrome on your phone, suddenly they can track you when you use their mobile Apps etc. And Apple is trying hard to grow in the "Services" field, i.e. through Apple Music and Apple TV - both available to Windows and Android users too. Just as they made a buttload of money with iTunes and the iPod because they also targeted Windows users.
