Wrong question, why it's not apples iphone, they sold it to you. And as a user you might be interested in a browser from a company interested in you using the web and not being stuck with apps that only work inside the walled garden
This is a good point, but we need to be mindful of the consequences. Opening the walled garden effectively hands the web over to Google, despite any immediate benefits there might be for users.
In the current situation, iPhone users can at least switch to Android. I'll take an imperfect status quo over a bad faith actor determining the future of the web for everyone.
Furthermore, why would Apple want an adtech black box on its devices?