People aren't rational/perfect and Signal wants to keep them feeling safe? ¯\_(ツ)_/¯

(and IMHO there are edge case scenario where the additional friction in exporting messages provides some protection. Particularly when your threat model involves imperfect actors)

edit: here's an example. Let's say I use 4 week disappearing message with everyone I chat with. That's imperfect of course, but let's say right now only about 5% of the people I chat with are proactively backing up/screenshotting my disappearing messages and the rest let messages expire. If Signal rolled out an "export all messages to plaintext" feature, then suddenly that 5% might become 50%. And now a lot more of my messages which used to disappear, are being preserved.

If everyone I chat with is a perfect 'threat actor' that always backups up every message they ever receive, then there's no difference at all. But most people aren't, so practically there's a big difference because now exporting to plaintext (and bypassing time restrictions) is trivial for the masses.