Because "the web" runs inside a browser sandbox?
Phones are with you constantly with GPS, movement, cameras, and microphones regularly accessible by apps, sometimes persistently. It’s also a platform handling SMS and Bluetooth and other weakly secure protocols.
That doesn't mean it isn't a sandboxed compared to more traditional computing environments - it just means it's less restrictive than a browser.
Why are people so obsessed with "forcing" Apple to adopt the same approach that gave us the dumpster tire fire that is Windows?
If you don't want a device where the hardware and software were designed to work together, just buy from one of the literal thousands of other vendors who took your approach.