most software available is not open source. Any app you download from the iOS App Store might not even match what an entity claims to be its open source repository. Any app or service with a backend of any sort that you don’t host is not auditable by you.

Self hosting is fantastic, but it is 100% not common and especially not something most people are interested in setting up. So your framing that this is objectively terrible either applies to nearly the entire software industry or isn’t a fair framing.