Why should information on Signal be ephemeral on Signal?

Signal is one of the few "privacy-first" services. But it is not just about privacy, it is also about having as many users as possible, which is actually important for privacy. If only people who really have "something to hide", then just using the app makes you a target. If instead it is used by millions of people for grocery shopping and dinner planning, then whoever really needs the privacy features will not stand out. The third is, of course, making money, because, of course, none of that is free.

That's why features like backups are important. Many people want them, maybe not you, but Signal is not just for you, it also makes money. By the way, that's also the reason why there are some privacy compromises, like contact discovery and the use of phone numbers, because they feel like done right, it is worth it. Note that have partially addressed both of these problems.

My friends and family chats are on Signal because they're seamlessly cross platform. Not feeding Facebook is just a bonus.