If you're doing a PhD, you probably want to make a difference in the world.
This is a good thing. It's good to have a goal that's bigger than yourself. But it can also be a trap if you try to fix everything you think needs fixing. By trying to fix everything, you end up finishing nothing.
Even those who have revolutionised their fields haven't done so all at once. They've done so by patiently and persistently addressing one problem at a time.