One set is costs directly related to employing you. Benefits, employer paid taxes, equipment, insurance, office space, a fraction of your manager, and so on. These alone are hefty.
A second set is buffer to pay you when you aren't being utilized. A well-run consulting company might expect 80% utilization, so 20% of the time you're incurring salary plus all the above costs and they are receiving absolutely nothing for it. A more typical consultancy might have even lower utilization rates.
A third set is the costs of customer acquisition and account development. There are expensive staff who do a lot of expensive things solely to get the contracts signed in the first place. And if a consultancy stops attempting to grow, it's at grave risk that a few existing customers will leave for one reason or another, and they'll be left in a terrible spot.
A fourth is the cost of finding somebody like you in the first place. If I'm hiring a junior employee it might be something like $10k direct, plus the time associated with sourcing, vetting, and interviewing candidates. For a senior employee, it could be a lot more than that.
Put all of these factors together, and the profits simply aren't nearly as hefty as you'd imagine.