If you use a catch-all term like "content" when you could've said "prose", "video", "music" and so on I'll assume you either don't know about the difference or you don't care - or indeed both.

The same goes for "IP" when you could've said trademarks, copyrights, patents, trade secrets and so on, whatever is actually relevant.

IP is the standard way of describing pre-existing content e.g. games, books that is available to turn into movies, TV shows etc.

You can go on and on about whether this is accurate or not but no one cares. Because this is the term that the entire industry has adopted because getting into the specifics of trademarks, copyrights etc is irrelevant for most parts.

If it's a standard you should be able to able to point people to the standard specification document/laws or etc.

Else it's just a habit some people have. (and potentially a lazy and problematic one.)

Games have prose and video and music and characters and worlds and proper names and the list goes on and on and on so we just say IP.