Businesses have the incentive to give their users that low friction experience (at the point of need) using already familiar rails (i.e. "install app from app store").
The makers of both iOS and Android treat the ability to "bookmark" a web URL onto your home screen as a power user feature that requires navigating through complex, technical-sounding menus. Does it have to be like that? Of course not. They just have a business interest in pushing users away from the open web and towards their walled gardens.
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Mind you, I'm not saying, "advertising doesn't play a role in this". A clump of well aligned motivations is obviously going to be more powerful than a single isolated motivation. But let's not forget that apps built for non-technical users, which—I cannot stress this enough—IS MOST USERS, benefit greatly from lowest common denominator solutions where they never feel like they have to learn anything to get going.