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.
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.