Apple doesn't believe the App Store is the driving factor for users installing applications.

They believe the App Store is one part of the total value of the iOS platform, and one which they chose to monetize.

And while the Mac App Store is a solid counter example to "do users value this?", the other counter-example is the Play Store, where, despite the availability of side-loading on Android, the Play Store is still, by far, the preferred way that users download and install applications on their Android device.