Yes, these companies sub-contract, but that doesn't change my comment. Sub-contracting is something that Deloitte et al know how to do, and have existing relationships with staffing agencies, have an existing management structure to manage those subcontractors, have existing contracts so you don't need to waste time negotiating, etc.
And with all that said, companies like Deloitte do have hundreds of people ready to go. They may not have a deep bench for certain niche projects, but I used to work at a Big4 and for something generic like "build us a website for people to sign up", we had entire offices of people around the globe in our offshore delivery centers just waiting around for work like that. Oftentimes we would still subcontract because it works out to be cheaper if 10-20% of your resources on any given project are subcontractors, but they weren't always necessary.
And with all that said, companies like Deloitte do have hundreds of people ready to go. They may not have a deep bench for certain niche projects, but I used to work at a Big4 and for something generic like "build us a website for people to sign up", we had entire offices of people around the globe in our offshore delivery centers just waiting around for work like that. Oftentimes we would still subcontract because it works out to be cheaper if 10-20% of your resources on any given project are subcontractors, but they weren't always necessary.