Surely you talk to more than one or two people, no? If you have to manually check a box or something every time you start a new message with someone, people are going to stop doing it.
Maybe you get acquainted to 100 new people every day, so your day is a constant chore of starting secret chats all the time. I don’t. I doubt regular people do. Just start the secret chat once and then pick it up later.
Signal is an encrypted chat app.
Except for the locally stored data which is not encrypted and Signal’s attitude is that device encryption is up to the user.