Because Apple cripples their browser on iOS, so you cannot make a web app. Basic app features like push notification are not implemented on iOS Safari.
Its a trade off between new and exciting technologies like the Push Notification and privacy - the service worker needed to provide them.
You’re free to send emails or text messages from the backend of your web app. This is hardly a great example of “Apple crippling their web browser.” Personally I don’t want your advertisements pushed to me. Even the notifications permissions prompt on desktop Chrome is ridiculously spammy since it’s on basically every commercial website these days. Safari on iOS is a great web browser that provides a great web browsing experience in my opinion.
So if I make a chat or video conferencing webapp you want to get an e-mail every time someone replies to you or invites you to a call?
I would love to take this energy for fighting apple on the 30% fee and use it encourage them to adopt web notifications and other web apis.