Er, because that would be really silly.
I like reading HN from time to time. I would never install an app, because I don't use it frequently enough. I definitely would never go through the pain of installing a HN app every time I wanted to read HN. I really doubt I'm alone or even abnormal in that regard.
That's the beauty of a browser: I can be reading HN in under a second when I want to, with no cluttering of my desktop just so I can read HN from time to time.