My best guess is that some usage of AI might be more subtle and not as obvious at a glance compared to say, visual assets. Like writing, coding assistance, music, etc. Additionally, the tag from selecting things in the dashboard is invisible, and split into several categories alongside the all-encapsulating AI generated tag.
For example, the game you commented on has AI generated code as you can see it appear here high up on the list: https://itch.io/games/tag-ai-generated-code