What is a phone call, if not "a conversation that self-destructs after it's done"?

Not commenting on how the law (which was written before "apps" existed) treats it, but on the true nature of a phone call vs. a messaging app.

There are many many records of phone calls. I can ask your phone provider for them. I can subpoena you and make you testify to their content, etc.

If your specific conduct is under investigation, sure.

This stuff is more about opposing counsel searching for keywords and finding something that adds supposed context to whatever they are suing about. If you send an inappropriate picture of a woman and make some crude comment, that can be used to bolster a harassment claim or establish a pattern of boorish behavior. If you make crude comments on the phone, that isn’t going to as easily reappear 5 years later.

I believe the difference is intent--a phone call self destructs by default. They are not meant to be recorded (we even have laws here that prohibit it without consent of both parts). Messaging apps have, traditionally, kept logs, in both ends and in the server.

Entirely agreed, though it is true that intentionally-private messaging apps have, traditionally, not kept logs. That's a tradition worth keeping!

No, that is why you see the legal disclaimer--either attorney work product or "privileged communication".

If you are talking with a lawyer, it's different than talking with a non-lawyer