I think there are two non exclusive ways of doing research successfully. Intense work and collaboration (in particular supervision of students).
PhD students and postdoc fall in the first category. They work full time on their research and do little else.
On the other hand, most successful experienced researchers are busy writing grant proposals, teaching, attending various committees. There's not enough time left to do meaningful research by themselves (plus getting older doesn't help to stay sharp and passionate), so they end up supervising students that to the bulk of the research.
I think someone that would try to do research on its spare time won't have enough time to devote to it (unlike a phd student), and won't be able to supervise students (unlike a professor).