You talk about the button Scan install locations for games, do you not? This is for games installed by the Itch app, for instance if you manually move the install location.
I suggest you visit https://itch.io/board/12011/itch-app-development , maybe there is more information. Also here https://itch.io/docs/itch/integrating/quickstart.html and most importantly here https://itch.io/docs/itch/integrating/manifest.html
My best guess is, that it might be possible to add games that have a manifest in their directory. It is not mandatory and I have no clue how many games do have a manifest. But chances are, if you can't install the game with the Itch app, it has also no valid manifest to begin with. Those games are meant to be uploaded by the butler app and therefore are hosted on Itch and not a third party hoster.