iChat stored every conversation (if you enabled history) in HTML files the user could find in their Documents folder and read. Messages history is stored in an undocumented SQLite database that you’re not meant to touch. I’m not saying they’re completely proprietary in every way, but you can see the progression from wide open to “keep your hands off.”

For another example, Apple Notes was actually just mail messages stored in IMAP, until they decided to deprecate that in favor of another iCloud-backed black box in order to add more features.