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.
Why do people not give away that neglected guitar/motorcycle/boat that’s been sitting unused for free?
Because one day they’ll get to fixing it up surely, and besides they’ve sunk so much time and money into it that it must still be worth a lot.
I understand your point XD but want to emphasize, software is NOT like tangible items, and any analogy to them will fall flat.
Giving out the source code to an abandoned application does not mean the original owner doesn’t still have their copy.
By just dumping the code online, the software has the potential to be worked on by other people. This can benefit everyone, including the original developer.
Even so, there’s plenty of valid reasons to not do it. Licensing issues (did anyone else work on any part of the project using a different license?), pride (no longer being the “owner”), getting it online (choosing a license, getting it online, it all takes at least some time and effort), or just plain “I don’t want to” are all valid reasons.