There are two reasons for this:
1. No one sits in an occupied chair.
2. Second guessing the boss is widely perceived to be a career limiting behaviour.
Enter the... Consultant -
Transient - you have to take what they've got and run with it. In the absence of a forever expert, folk solve for themselves, for their unique situation and take ownership.
Also - there's no fear of a consultant messing with anyone's career; consultant bashing is an international sport, and the minute they're out the door, you can blame them for everything.
So - difficult transition looming? Want to shake things up? Got some new ideas? Use a consultant and get a get out of jail free card for free.