Why is the add-on I purchased for Fortnite on Xbox not shared through Xbox game sharing?

Fortnite add-ons often contain game-specific contents such as cosmetics (outfits, pickaxes, and other stuff only available in the game) and V-Bucks (in-game currencies). These kinds of add-ons are not always shared through Xbox game sharing. You can check this for more info: How home Xbox and game sharing work (Opens in a new tab)