The proposal went through revisions. The fee changed after a call. A client constraint came up in a meeting. By kickoff, the signed agreement is in the folder, but part of the thinking behind it lives in a principal's head and an email thread the team never saw.
The project manager is capable. They simply don't have the same picture. So the principal stays involved in every clarification. Do that across several projects and it takes a real bite out of the time available for clients and design.