There's a difference between strategy consulting (e.g. McKinsey, BCG, Bain) and consultants that do large implementation projects (e.g. Accenture).
Yes, it's not so black and white these days as both do a bit of both but the reasons they are hired are very different:
Strategy: best practices, ass-covering, politics. Smaller teams, shorter timeframe, C-level. Think interviews & powerpoint slides.
Implementation: body leasing, offloading large projects. Larger teams, IT-heavy, longer timeframes. Actual coding / documentation, etc.
And no, your experience working on a 300-people core banking project at Accenture doesn't translate to a 5-people McKinsey project.
And while both types are "expensive", they are needed in the corporate world. Because work in that environment is more than just writing code in a text editor, it's mostly politics at the top.