Earlier quoted context omitted.
Because it makes apps more expensive.
If the apps become more expensive then people will stop buying them from the App Store and seeing people are not willing to pay for more expensive apps developers would probably stop developing for iOS which in the end would cause users to abandon the platform seeing as they cannot get the apps they need. If apps being more expensive is really the only problem then it seems like the problem fixes itself.
You're assuming efficient markets. The App Store is a monopoly, i.e. the opposite of an efficient market. (Well, it's more nuanced, all markets are somewhere between efficient and monopolistic.)