The pressure of potential PhD professors to obtain grants is instilled from early on. Then, the pressure to maintain grant flow into a university stays in the PhD career. In order to be a professor, one must compete with other potential professors by displaying how they will bring in the grants. That's only one of the things that goes on behinds the scenes, not to mention the stacks of paperwork and research grant proposals. A professor often has some wiggle room to choose what to research, but that is often determined by WHO/WHAT is handing out the grants to study a specific topic. How do I know? Well... friends that want to become PhDs tell me.
It's like a professor is a leprechaun afraid of losing that pot of gold at the end of the rainbow.