in my experience, it's common in academia for code to "float around". often it's not officially open source or distributed, but you know someone who has the code for some paper, so you can use it. there's even a few textbooks in this state (for years).

i think the real reason for not releasing this code is time---you might be surprised at just how much effort goes into making a paper/talk/article polished. sure, you could just release the hacky code you used to do your calculations, but you'd like so much more---for it to be useful, for it to be a credit to you, for it to also do XYZ. research code is efficient in that it gets exactly what you want for the paper; it's rarely usable by anyone else. there is essentially zero payoff in academia in publishing code, and so there's no time to fulfill these desires.