So to make development faster and make sure they didn't waste time, they spent their time reinventing slack/teams/SharePoint/etc.
It sounds like if Nintendo were a person, they'd have ADHD. This also explains how for every generation, their flagship console looks like a completely new thing. They're just getting understimulated and bored.
Looking at the slides in the original Japanese source, this tooling also has a whole lot of analysis options and can pull/push game data/positioning both to and from a real Switch or something along those lines. Integrating that much custom features into an off-the-shelf tool would probably take just as long.