Most PhD topics are incremental, because you are supposed to do a PhD very early in your career. Because the American system won and the PhD become the terminal degree. Which you often do as a student rather than even a junior professional.

In my experience, academic researchers are more likely to do significant independent work in their 30s than in their 20s. Some academic cultures have higher doctorates, habilitations, or similar milestones to wrap up this period of peak productivity, but those remain national oddities.