Rewriting would introduce new bugs; it would take a large number of engineering hours away from delivering shiny new things; and a formally correct version would probably be less power-efficient.
It won't happen because these targeted attacks don't affect the bottom line whatsoever. Nobody is switching to Android just because a journalist or NGO employee occasionally gets pwned.
It doesn’t really matter if there are 100 new bugs for every memory unsafety bug fixed. Those new bugs in an image codec would be hangs/crashes or incorrect rendering and that’s it. And that might be serious but it’s not a security vulnerability.
It won't happen because these targeted attacks don't affect the bottom line whatsoever. Nobody is switching to Android just because a journalist or NGO employee occasionally gets pwned.