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.