"Easy" does not mean "lowest cost" or "easiest". It's far far far easier to stor conversations as plain text and return them as is, instead of having to encrypt, rotate keys, etc. etc.

That's a tricky system to get right and maintain

(Please do not interpret this as a defense of OpenAI! I just think that we shouldn't trivialize the task of encrypting user data so that it's not visible to the provider).