Because it's Apple's incentive to offer a good web browser to sell very expensive devices? The same incentive Microsoft has to make Edge and Google has to make Chrome for Android. They do not have a god-given right to make money on every piece of software that happens to run on devices they sell.

Besides, this wouldn't be an issue in the first place if other web rendering engines weren't forbidden on ios. Google / Mozilla could have their own browsers that aren't just safari reskins.