Because essentially 100% of web pages that request push notification access are garbage tabloids that nobody cares about. No push notifications on mobile Safari

Because essentially 100% of web pages that request push notification access are garbage tabloids that nobody cares about. No push notifications on mobile Safari means that there is no incentive to build mobile web apps that function similar to their native counterparts. Obviously people are accepting push notifications somewhere or else they wouldn't be such a popular design decision.