Hi Jessica, thanks for your feedback. We understand your frustration. The Mac and the iPad App Stores are separate stores and developers need to upload different apps on these stores. A lot of apps, that are either free or require subscriptions allow you to download the separate Mac app for free or include it in the subscription pricing. This is not possible for paid apps like GoodNotes and also Notability. The creators of Notability also sell their Mac companion app on the App Store as a separate purchase for the same reasons we explained: https://support.gingerlabs.com/hc/en-us/articles/206088327-Buying-Notability-for-iPad-iPhone-and-Mac

Apple’s recent developers indicate that the platforms will move closer together, so we hope that they will eventually also allow developers of paid apps to offer a single app across all stores or at least offer a bundle. Subscription or free pricing is currently not an option for us.