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.