Why didn’t Wordperfect or Corel make their source code public, or lay out their document formats as public domain for the purpose of standardization? You’re wanting proprietary software vendors to make it easier for other people to take their customer base. I can see why you’d want that if you’re in a position to be one of the people who’s trying to take it, but I can’t say I particularly blame them for not wanting to.
Microsoft learned about the power of proprietary lock-in from Wordperfect.
You’re wanting proprietary software vendors to make it easier for other people to take their customer base.
This is one of those cases where what the company thinks is best for it may not be aligning well with what's best for the customer. Format lock-in means you have less need to rely on the quality of your software to keep your customers.
Notice I haven't said anything about forcing Microsoft to make Exchange more interoperable. But its lack of interoperability (plus CALs and that monstrosity of a database) are very good reasons to avoid it or leave it if possible.