By contrast, Apple doesn't have a monopoly on the mobile device market, the browser market is dominated by Google's Chrome, and the majority share of browsers use Chrome's Blink engine.
'PWA' is a Google-invented term to describe technologies already implemented in their own browser; none of it constitutes a single standard unto itself. Apple not fully implementing Google's standard, made not for the benefit of the open web but to make ChromeOS a little more functional than a glorified web browser, is only a threat to those who've forgotten that coding for one browser is what got us stuck in the code-only-for-IE mindset and halted progress for browsers for about a decade.
Apple isn't the biggest threat to the free and open web. Google is. Everybody's using Google's browser, based around Google's own standards designed to force people to stay using Google, to access Google's search which displays Google's ads above Google's curated AMP results.
Whether you agree with these decisions or not Apple is clearly and deliberately using its influence to limit what a web app can do.
[0] https://developer.apple.com/notifications/safari-push-notifi...
As to why not mobile Safari, perhaps Apple doesn’t want to be tied to limitations of the design?
Apple’s push notifications on mobile have evolved significantly over time.
Just because they could do something, doesn’t mean it is a good design.
Not so with websites. The bar for including new functionality in browsers is way higher than it is for native apis because there is no way to police abuse.
As it is, developers and management recognize that Chrome !== Browsers, which means that they also budget in Safari, and sometimes FF.