The gentleman behind PCalc noted that when he ported his iOS dice rolling app to macOS using the checkbox in Catalyst, the resulting app was not very "Mac-like" and he had to do a fair bit of massaging to the code to make it look and act like what would be considered normal (even acceptable?) for a native Mac app.
Marco Arment has also noted that his macOS version of his iOS podcast app (Overcast) is taking a fair bit of effort to make it work like a native macOS app, as well.
Since most of these developers will likely be charging money for their apps, they likely want them to look and act like native macOS apps to be acceptable to macOS users and it sounds like a Catalyst 1.0 direct port is not anywhere near that level of polish, as of yet.