Think about the question this way, everything you do on a project costs resources or money, be it planning scoping etc.
Therefore companies before they commit to resources , even to go through initial planning phases, need to see the business case before they put in investment of people.
I am actually going through this right now in my job, we are trying to get together a solid business case and ROI, to get the leadership to buy into resources and the planning phase.
Scoping a management plan would require resources, people would need to sit around a table and in many companies they would have to clock their hours against this project. However the business case has not yet been made to say its worth investing time and effort in, before buy-in of the ROI from the leadership.
Hope this helps