Why do people not put their code on github even after publishing?

I am from an interdisciplinary field and have observed that even after publishing a paper they don’t put their code on github. They have a statement like code can be shared if requested. What could be the reasons for this?
I just wanted to look at the code and see how they did something. But I need to mail them and they always say they need to clean it, so it will take more time. But it had been many months their paper was published. And now they know, I am the one looking for it. I don’t like this at all.