Signal's default setup is more usability focused while supporting E2E, and less about tinfoil hat threat models about being present on a continent you're a citizen of.

The items you mentioned can essentially be configured, for those that want the insane level of privacy / security. Messages can be auto-deleted 30 seconds after being seen, a proxy can be configured to route all your traffic through it, and tons of other things can be done to customize it more to the user's liking.

I'd imagine they're caching it because of egress costs. File attachments, voice mail, video, etc. can all add up.