Lack of it is the root cause of most failures.
During the planning phase, nothing of value has been produced yet, and stakeholders are eager to get something for their support. Last thing they typically want is a bunch of planning; they just want results. So most project managers fall prey to short-cutting the planning, which leaves big gaps that will certainly plague the project downstream.
Of course, when Planning is done (woops.... it's never done) the next most important phase is Execution. Some might say Monitoring & Controlling, but if properly planned, then execution prevents the issues that would require monitoring & controlling. At least that's the theory, and a philosophy worthy of pursuit.