I don't know the details first hand, so take this with a grain of salt, but this is what I remember from talking to engineers who explored this possibility over the years:

You can't ship a native JIT (execute native code from the heap), I think it's enforced automatically by the App Store. So shipping your own JavaScript engine is out.

You can't download and interpret third-party code, I think this is enforced by human reviews, so shipping your own Web rendering stack is out.

You can use your own network stack with WebKit on top, which would let you offer some value to users like more advanced ad/tracker blocking.