Slack works just as “well” on the browser, so why should I take a considerable performance and battery life hit for a webview, when the dedicated webview already provides all the functionality I need?
As for Slack, I'm not really a fan, but I use their app because its easier to find with its own application layer compared to being one of a hundred browser tabs I might have open in my various web browsers.
(That's the same reason I use Epichrome to make dedicated Chrome-based browser apps for GitHub, CircleCI, JIRA, and some other tools... macOS gives you a slew of useful tools for dealing with things at the application level. You can absolutely do all the same stuff in a browser, but it's not as convenient when it is intermingled with all the other stuff going on in your web browsers.)
I'm a macOS developer, and there is no way I would ever use something like Electron to make a Mac app — it would be inferior, in all sort of ways, subtle and otherwise.
But there is also no way I could create a great native Mac app, and a great native Windows app, and a great native Linux app, unless there was a relatively massive development budget (of time, which generally implies money).
You shouldn't be comparing Electron apps like Slack to the nonexistent high-quality native Slack app, which Slack would never build, and therefore exists only in your imagination.
You should be comparing Electron to other frameworks that deliver cross-platform apps across the major desktop platforms: basically, Java and Qt.
I run Slack and VSCode all day; no, they aren't as nice as good native Mac apps are, but they are about as nice as the Java-based JetBrains IDEA tools that I also use.