It may not be clear who holds the rights to the source code. The developer may not have the source code anymore. Even if they do, they may not have the rights

It may not be clear who holds the rights to the source code. The developer may not have the source code anymore. Even if they do, they may not have the rights on third party libraries, third party source code, or digital assets. Releasing source code has risks. Source code can include profanities or use terms that nowadays are considered inappropriate, or patent holder may inspect it to find patent infringements, and sue. Most of the time, there’s very very little benefit, if anything at all, to the developers in releasing the source code.