It is a user experience choice. Most decent native macOS apps adhere to system conventions. Text editing works as one would expect in a system text field. Keyboard shortcuts behave as expected. Custom keyboard shortcut definitions from the Keyboard section in Preferences actually work. And on and on.
Slack in particular is extremely, extremely slow, even by Electron standards. I can't expand it to the full-height of a 4K (running at 1080p) portrait monitor without typing becoming unbearably laggy, so I either have to keep it at 1/3 monitor height, or have to use another application for writing the text.
it also has multiple accessibility issues for low vision users. I'd happily switch to a native client because using native widgets instead of HTML would likely address most, if not all, of my accessibility needs.
