Nobody’s stopping you from using the web version of Slack or Spotify.

That said I can think of some pretty good reasons.

- Users don’t understand it as well or are less comfortable with it.

- Users’ general preference for logical separation

- Mobile safari doesn’t support browser notifications.

- You won’t get browser notifications if you close the tab by accident

- You lose OS integration, like replying via OS notifications in Slack or using OS Music controls with Spotify

On top of all that a browser tab uses plenty of RAM as well so I don’t really see the upside of using the browser for these apps, either.