It's just that OWA is a web app that is often configured to be accessible on the internal network via an internal IP, and on the external network via an external IP (so you can get webmail even if you're not in the office), but it's the same app.
I certainly don't think it's worth supporting this from first principles - i.e., had the web been designed the way you suggest from day one, that would have been great - but it's got a very large deployment, and "Chrome silently loses your Outlook emails because of a security issue in video games and a BitTorrent client" is a good way to get Chrome removed from enterprises, and other browsers to decide not to ship the same fix....
