The average users sucks at managing bookmarks.
Telling a user that a feature is a webpage instead of an app means that they, in their glorious tech illiterate brain, think they have to memorize the url. They don't want to do that. They want to "have it" so they don't forget it. And having it is easier in the app mental model to them than in the bookmark one.
I know sufficient 40+ year olds who have the most absolute disgraceful phone desktops, filled with garbage. And yet, this is what they want. They want their bookmarkrs in the one place they'll check. And they want no folders.