Wow! One question I have wanted to ask for a long time - why is the source code for old games often released under a free license, but artwork and other assets

Wow! One question I have wanted to ask for a long time - why is the source code for old games often released under a free license, but artwork and other assets are not? What are the incentives of the publisher in this case and why are they different for the source code and for the asset?