Why didn't Microsoft make Exchange compatible with the earlier WordPerfect Office? Because they're all proprietary, practically designed for lock-in. Standards are supposed to allow everyone to use them. Microsoft has a long-standing problem with that concept.
You seem determined that Microsoft has some obligation to make it easy for everone else
No, Microsoft has no such obligation except for certain aspects of its criminal monopoly conviction. Of course, nobody has an obligation to buy Microsoft's products (I hope).
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.