[Zak]

Slack and Discord work fine in desktop browsers and do not regularly hassle users about installing a desktop app. I think Discord users often install the app for actual user-centric features that are not possible to provide from a browser (push-to-talk voice chat while playing games for Discord, as I recall).

[TheCoelacanth]

I don't think either of those apps are ad-supported, so there are completely different incentives.