If pinning website bookmarks was more prominently advertised in the UI, they would be fine with it I guess. Though it would need some more changes.

Why do people like apps? Because they can put it on their home screen, they can open the app list and pick from there, they are searchable in a canonical repository, which is kind of like googling for the website but still.

Login flows are simpler and persist better, with local storage etc.

Multiple apps can be switched between by just moving between the currently opened apps, while website tabs appear inside the browser only and are mixed with many other unrelated browsing tabs, making it harder to find.

I guess fundamentally all of these could be supported with browsers. But in the end, Google and Apple don't want to make bookmarks and independent persistent "browser windows" easier.