What advantage would that introduce? Slack seems to work just fine on each available OS. Being able to write once in JavaScript/CSS/HTML seems a lot more appealing than writing and testing three apps separately, regardless of your company size.
As a dev I'm biased of course and this might not matter to most people. One thing though, you don't have to write the full app three times. For example all the business logic can be in C++ and write a native UI on top of it.