Replacing libjpeg, libpng, h264 & h265 codecs etc. is a gargantuan task. Even if Apple employs another 200 rust programmers (which don't exist in the market – so not possible) it would take years before that project is close to finishing. So intermediate solutions are necessary until then. It is also likely a rewrite would introduce other security issues (not memory safety issues) which would take time to fix.

Rewriting these libraries is probably also a common good, that would be better done through open source initiatives.