Why aren't outbound group sessions persisted in the store and reused accross app launches?

I've been studying the Riot iOS code and noticed that the group outbound session is recreated for each room on every app launch and then cached for the current run.

I'm wondering why aren't the outbound group sessions persisted to disk and used for longer?

I mean there's a noticeable overhead of always sending the room key for a new session after each app start. Is there a reason why this approach was chosen?