iOS safari users have made up at least 50% the traffic of the last few sites I’ve launched. In the US at least they effectively have veto power on new browser features.
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.