the perception is that employees have an incentive to lie: to cover their ass, to cover their incompetence, to be or stay lazy, to ensure their bonuses are better, etc.
in theory, an expensive outside consultant does not have those incentives: they are being paid 800/hr, have no dog in any fight, have a fixed contract and no reason to worry about getting promoted or fired next cycle, and have already been pre-vetted for expertise.
if I do not have faith in my goober, fork-and-spoon operator employees, I pay for these folks to validate the approach. that's not incompetence, that's due diligence.
it's not my money, nor my department -- it's the shareholders -- and I need to convince them that I've done all I can to confirm we need to invest X million in initiative Y.
however in practice consulting falls victim to plenty of bad behaviors, rent seeking, and risks, too