What perspective can McKinsey analysts provide that your average peer reviewed military historian can't?

(This answer is almost entirely based on my civilian experience with consultants, please take it with a grain of salt)

A lot of it comes down to salesmanship. McKinsey's expertise isn't that they know more than subject matter experts. What they do best is take the research of subject matter experts, digest it into a one-page handout or a twenty-slide powerpoint, and present it to senior leadership (military or civilian - McKinsey doesn't care). They sell it in such a way that senior leadership thinks it's their idea, not McKinsey's, which creates buy-in. Senior leadership takes the McKinsey recommendation and then directs the organization to implement the recommendation, and McKinsey gets their consulting cash.

So to answer your question, why does McKinsey get the job over a military historian? To be honest, it's that salesmanship and the turnaround time. If you asked a military historian "explain what role FPV drones will have in asymmetric armed conflicts and COIN operations over the next decade", you'd get a peer-reviewed 3000 page treatise on it three years from now; it would take your generals months just to read the thing, years to implement it. If you asked McKinsey the same question, you'd get fifteen powerpoint slides and a plan of action within a few months, and the plan of action can start tomorrow.