Some people are plenty mentally stimulated at work and don't feel the need to work on their own projects in their spare time. And sometimes your professional accomplishments are significant enough that a side project is not likely to make you a much more attractive candidate. The people who "do both" often don't fall in these categories.

Also a lot of companies don't want their employees to write code for anyone else and will actually see open source contributions as a major red flag.

Good point.

But if the code has copyrighted issues, then how can I show publicly? I usually have a copy of repository for my past projects and I don't mind to show it to people who interview me.

Also I think if I need to put code on GitHub, it has to be some software for more generic tasks.