[nullc]

The primary choice there is that loss of your password doesn't mean losing your private messages. There are even ways around that, but lose your password lose your private messages isn't a terrible tradeoff particularly given that these platforms are fundamentally unable to provide the level of privacy the users casually expect. Especially since direct messages are usually used as a replacement for completely ephemeral communications (like phone calls or watercooler talk) that historically would have left not persistent record of their content at all.

If the private messaging were encrypted by default it would also be easy to offer an opt in feature that would preserve recovery (e.g. by the users software encrypting a copy of their encryption key over to the platform). I expect relatively few users would opt into it.

Imagine that back in the 80s when people regularly communicated by letters in the post if the post office offered an option to open, copy and archive all your letters, just in case you lost them. ... how many people would have opted into it? with the obvious invasion of privacy, vs just taking care to not lose whatever they intended to keep... Not zero, I suppose, but not many. And today these platforms archive and hand over communications far more casual and ephemeral than letters.