I just uninstalled a few games from GOG and it left a bunch of files inside the folders. I wish it would clean them up.
That’s just windows, the uninstall rarely removes everything from an item. Have a look I programdata, %appdata%, and other areas. Some applications, not games so much, are specifically designed to be hard to remove, leaving things all over. It’s a reasonably small part of housekeeping though. Minimal installs, cleanup once in a while.
It can sometimes be beneficial, such as the save games mentioned above, I once lost a load of in game pictures as I uninstalled which wiped all.