The current need to create them outside of the browser feels unnatural (with file browser analogy in mind), when the same context menu allows you to add notes (files) but not decks (folders).
Related:
Some context:
My actual user story is that I started without the need to create decks for multiple weeks. I found a deck for the language study book that I’m using and worked with that.
Only later I needed to create new subdecks, to reorganize the cards into subdecks and create new decks for lessons not included in the shared deck.