There are many closed source applications that have been removed from the Google Play catalog for various reasons.
Often this reason is the cessation of further development and maintenance of the application.
Why don’t these people make the code open to everyone if they don’t need it anymore?
This could bring great benefits to all people, but everything that was once created simply disappears.
It would still be nice to look at how someone did something even if implemented poorly to make redevelopment quicker.
As for why people don’t do this they might just not care or believe they will maybe get back to it again someday even if that is years in the future.