It's not uncommon for people to put additional effort (that you can spend on a 20%) to fix stuff in the primary project. Like, voluntarily adding themselves to a build cop rotation or doing code / readability reviews as a service, and so on. And sometimes, the primary project is tight on deadlines and people want to stay focused.

And even when a person is enjoying the 20% opportunity, most of the time there is no good project to take on. During my stays at Google, I've done many 20% projects (>10), but only had a 20% project may be a third of the time.

People enjoy their work in different ways. Some like being 100% on their main project, some like involving themselves in many smaller efforts (both across their main project and across 20%). Also, the default state is to not have a 20% project, the only people that will work 20% on something else are those that made a specific effort to do so.