Not everyone has or needs an extensive GitHub page. I'll use myself as an example. I've got over a decade of experience working in large teams to ship commercial products. As a general rule, you won't ever see my code unless you also work for the same company. The kind of companies I work for want me for the experience from my day job. I can easily prove I worked on the projects I worked on but any company interviewing me would have nothing beyond my word for what exactly I contributed to each project. It's totally reasonable for them to ask questions that can confirm I know how to do things I claim to have done.