Storage Management on Catalina displays nonexisting apps. How to fix?
From the Steam Knowledge base on Uninstalling from macOS:

Just dragging the Steam program to the Trash will not fully uninstall Steam. The additional steps below are required.


Open Applications and drag Steam to the Trash.

click Go in the menu bar, hold down the option key until you see the Library folder, and open that folder from the drop-down.

Navigate to Library/Application Support/Steam

Delete Steam data from your computer with one of the following options:

If you don't want to lose any game data: delete all files and folders in /Steam EXCEPT FOR the /SteamApps folder (this contains your game downloads and any local save files.)

If you're removing all Steam data from your computer: delete the entire Steam folder.



Empty the Trash by selecting Finder -&gt; Empty Trash


Reindex your Hard Drive
If Storage Management (like Steam) is just a front end to all of the system services and it just may not be updated.  If it is still showing that the Application is there, it may be required to re-index your drive.  In Terminal, issue the following command:
% sudo mdutil -E /