Installing an app is not a smoother experience. Allowing notifications is one click. Also, apps don't always have the same features as websites. You also make the assumption that services I use with browser notifications also have apps. This is not true in the slightest. And in some cases, I still prefer the website over an app.
How is it hard to imagine a service I don't frequent use but when I do, I want notifications? I use these services a few times a year at most. Why would I want to install an app if they have it?