Of course they could. They looked at the cost of rewriting the entire integration and framework for running PWAs and said, "eh, nah."
I have the feeling Apple is betting on Google not caring enough about the PWA platform to try to compete. Maybe they're right, but if they're not, they're only making the browser wars worse for themselves.
I don't think it's about Google, I think they assume consumers won't care, and they're probably right.