Currently, AnkiWeb is still though as a “companion app”, because it lacks many features with respect to Anki Desktop, while AnkiMobile can be used as a standalone. Over the years, since AnkiMobile has been quite popular, and since it’s currently the only sold part of Anki, AnkiMobile received much more attention from the dev than AnkiWeb. It was though mainly to review on the run, while the main interface to edit the collection would still be Anki Desktop, I think (at least that’s how I understood it when I first read about Anki).
Probably because the app, just as ankiweb, started as a companion app rather than full featured Anki Dekstop alternatives.
