Apple is providing an instantaneous, global distribution network.
Save your ability to market the app, you can effectively guarantee the same experience across billions of disparate devices. The open web can not provide the same piece of mind in terms of security to the end user.
The app vetting process isn't free.
Apple is ensuring quality within the App Store. Bad apps still get past Apple's vetting rules and are retroactively denied all the time. This creates extremely a strong sense of trust between the customer and Apple. Consumers become less and less risk-averse when it comes to downloading new apps from the App Store. Again, I'd argue this runs counter to the open web due to its long history of leaks, hacks, privacy violations, phishing attempts, etc. The Apple App Store currently has the highest bar for an "open" ecosystem of independently-developed software.
Apple provides the service of the App Store at its own expense.
The App Store has pulled in $100BB over 10 years which is great, but I'm sure they'd turn it off in a heartbeat if it consistently caused serious security breaches within the phone.
Tim Cook has gone on record so many times about the personal and private nature of the phone. Whether it's possible or not for a rogue app to circumvent Apple's numerous security safeguards is irrelevant as security is a policy Apple takes extremely seriousl; maybe to their own detriment.
Apple has a history of providing basic, free alternatives to apps that monopolize particular verticals within the App Store.
Calculator, Flashlight, the various flavors of Timer functionality, the new ARKit Ruler app, etc. I don't view these Apple-provided apps as akin to drug stores offering basic alternatives to things like Tylenol etc. it's good for the overall App market economy as it should stem the rise of local monopolies.
Ironically, the only app in which Apple does not offer an alternative is the App Store. The App Store isn't an alternative, it's simply the only way to download apps.
I do see a future where phones will have a heavily-reduced version of iOS that offers a small subset of Apple-created apps without the App Store. The numbers don't lie, most iPhone users do not download apps, or only download less than 10 apps over the lifetime of the phone.
At the end of the day, apps must be vetted for quality because Apple has a brand to maintain. They simply can't allow apps that seriously compromise the performance of the phone or the privacy of the data on it. It's not within the realm of an allowable reality as per Apple policy.
The 30% taken by Apple may be eclipsed by the App Store ad revenue. I assume they'll greatly reduce the 30% cut moving forward as the App Store ad revenue increases.