There's a market. Almost everyone is a market taker.

For instance, a company has to figure out how much to pay people. If McKinsey comes in and says "pay them less and we'll tell everyone else to pay them less" that just doesn't work. Why would any company that is 1 of N decide to pay them less when they can just pay them slightly more and attract the talent from all these other places. There is an incentive to cheat and without a legally binding agreement all collusion services will collapse. And this even ignores new entrants. You can have individual (illegal) agreements between companies like Google and Apple where they agree to not hire their employees, but those break down and have nothing to do with consultants (why would you want a middle man in an illegal activity?)

The reality is there is a market for most things. To hire an analyst it'll cost you X. McKinsey and other try to discover that and tell their clients. But they can't change the nature of the market as much as someone installing an HVAC unit can change your temperature but giving you a rigged thermostat.