That's a different model. That's generally called staff augmentation, whereas generally the bigger consulting firms get pulled in to help the business deliver on the AI agenda (or whatever McKinsey's selling to clueless execs this year).
In my experience, it's also common that those big consultancies get those impossible projects because none of the internal teams what to work on them. And when they inevitably fail, they'll convince senior management to pivot to something more feasible.
