I can determine whether an app is trustworthy, sure. But you know what? Sometimes I actually want to install an untrustworthy app. Sometimes an untrustworthy app is the only app that does what I need.

Your argument, by analogy, is “you should trust people to know not to sleep with people with STDs.” Well, you know what? Some people want to sleep with people with STDs. Sometimes those are their significant others. They still don’t want to catch something.

In both cases, the answer is the same: a condom.

A sandboxed App Store is, basically, a brothel where condom use is enforced. You can meet strange apps, play with them, and not worry about it. Because of the brothel’s policy, nobody the brothel hosts is risky. Your safety is enforced at the level of choosing the source.

Whereas something like Ubuntu’s PPAs, are more like a bar. Who knows what you’ll catch? Any individual app might decide to “wrap it up” with SELinux/AppArmor, but you can’t enforce it at the app-store level.

(Also, completely dropping the metaphor: the iOS App Store is frequently exposed—at least for free purchases—to children or even infants. This is actually a capability people want. This is certainly not a case where the user can determine for themselves whether an app is trustworthy.)