Because a user first encounters it on the web - where it acts exactly like a website. The only difference is that the bookmark-button-thingy says "add app to home screen" instead of "add to home screen".

It's not giving a "I'm installing an application" vibe, it is giving "I am creating a shortcut to a website" vibes. Apps are installed via the app store, not as weird quasi-bookmarks in your browser.