I suspect a large portion of Apple’s users value the fact that they can’t break their phone by accidentally installing the wrong app or clicking the wrong button. To them it might not be clear what they’re getting from this no-rules App Store.
It's easy to say "well that's just their own fault if they installed something bad," but I don't think Apple really cares to argue with their customers over whose fault something technically was. Apple just wants their customers to have phones that are working well.