They shouldn’t, and in some places they don’t. That’s the problem: different countries have different views on how permission should be granted. In Brazil, for example, you can’t legally give up certain rights over a work (they’re nontransferable, presumably to protect creators from companies with abusive contracts) and word-of-mouth is just as binding as a written contract.
For software, licences typically do two things: provide a clear description of how it should be used (giving permission to use the software without getting sued by the author), and warranties (protecting the author from getting sued). Simplified like this it all sounds silly, and it truly is, but in a world where companies sue each other over the silliest things the consequences can get serious. Ideally, licences wouldn’t have to exist at all.
Art rarely needs to be packaged with a warranty so software licences (especially the GPL) are an odd choice. Posting an artwork and saying “Feel free to use this as you see fit” is probably just as valid/binding legally as a text file written in legalese… depending on where you and the author are.
The legalese typically helps in making everything crystal clear and you shouldn’t have to contact the author. As an example: the Pickford Bros’ site leaves an ambiguity on whether the fonts should be used by non-indies. On OpenGameArt it’s common to see posts like: “I’d like to use this. How do I credit you?” A proper licence that requires attribution typically wouldn’t leave doubts on how to do so.