The challenge is that the notification acceptance rate is so low on mobile devices that it makes it meaningless to implement such features [1]. Practically, user is most likely to accept notifications from sites that they interact with a lot. Those also happen to be the sites whose app user is willing to install. Thus, the notification feature on web is not something that a lot of users require.
> This is a reasonable feature to exist on the web platform
Not really? [1] indicates that notification prompts actually result in users navigating away from webpages clearly demonstrating that this is a user hostile feature.
[1] https://blog.nightly.mozilla.org/2019/04/01/reducing-notific...