In this case I would recommend you to add license metadata. Read here how to do it
I proposed to make it less hidden but I don't absolutely agree with your sentence.
Most games use proprietary licenses explicitly or implicitly* (see the end of the message)
The similar situation with assets...
The default value of metadata in itch states to the "no license" that is in my opinion the same as "all rights reserved" but added to distinguish the "NULL" state...
When people use license they usually use their custom license (which are usually proprietary but it doesn't matter in this discussion). There are no predefined item in the drop down list so users usually write it in the description or just put a separate file with license...
There could be a partial solution. The common points of the licenses could be extracted and end user could compose from them in metadata custom license but the description is more flexible and robust ¯\_(ツ)_/¯
*Each work is copyrighted by default so technically without permission of the author you can't do with it anything at all