[ChuckMcM]

I think the author answered their own question with this statement: "I’ve been on flights where the sound of the flight attendants over the PA system was loud enough to sound like the attendant was shouting directly into my ear despite having two layers of sound protection."

The PA system in the airplane is part of the safety equipment, the crew uses it to inform the passengers during an emergency what they should do to prepare. Everyone knows that during a flight people will have noise cancelling earphones on and possibly dual layers of protection. Also the ambient noise in an aircraft with its nose pitched down at a steep angle or in an uncontrolled spin is likely to be quite high. The PA has to cut through all of that in order to communicate with you.

I agree it would be nice if they didn't use full emergency power during non-essential communication, but the FAA considers the safety briefing to be essential communication so you are out of luck there.