The legwork (mostly making powerpoints, from what I have heard) is done by bright, recent graduates from top undergrad programs. No doubt they are all intelligent and accomplished, but what value does a 22-year-old Harvard economics grad bring to (e.g.) a pharmaceutical pricing problem? Certainly not relevant experience. Probably not even "business judgment".
Teams are led by MBAs and some PhDs, most of whom will not have industry-relevant experience (though some will on certain engagements). I don't have an MBA, but two of my college friends described one top program as "a summer camp where you learn about Excel functions". This makes their advice desirable to large firms??
The breadth of knowledge which would be required of any single consulting firm to advise on the basis of direct experience across the range of industries in which consultants operate is staggering. They don't have it.
If what management wants is the consultant's imprimatur on an idea they want to implement anyway, why not either just commit to it without McK's 'approval' and save a few million dollars OR try to get an outside blessing for the idea more cheaply? I get that McK has a "pedigree" and this may be part of what the firm wants, but that's an extraordinarily expensive way to solve the 'problem' of not being willing to take responsibility for your own idea.