I'm talking about for use in Apple's first-party apps, like PassKit/Wallet, which is how the zeroclick happened recently. Apple gets to choose what codecs PassKit uses.
They also use them in Safari, AFAIK.
I'm also pretty sure most consumers of them are using them via ImageIO, which is under Apple's exclusive control.
Because they're shared libraries which other programs have linked to.
Additionally they've had decades of work and rewriting them increases chance that it will break something.