I think the easiest answer is that a majority of consumers prefer it that way, if you don't have an app you don't exists as far as they're concerned. Remember that a lot of students doesn't understand file paths[0], they're so used to smart phones that they've never learned basic computer skills. Add to that that even before smart phones a lot of people didn't understand URLs or even used bookmarks, they just googled whatever site they wanted, like facebook login. Having the app icon on your home screen is much easier for them, and they're more likely to use it again and not forget it.

[0] https://www.theverge.com/22684730/students-file-folder-direc...