Most tech companies invest in things such as culture, internal productivity tools, and developer and testing infrastructure that are not deliverables to clients and thus not billable. While these may incur a higher start-up cost, these are critical for ensuring long-term productivity.
Correct me if I am wrong but my impression is that the consulting world is much more concerned with delivering and then moving onto the next project, with very little shared work or infrastructure between projects.
I'm sure 18F has areas of improvement for how it spends its resources and how it tracks costs and billing. Every organization does. However, I would not be surprised if the reason 18F exists is that the government found that outsourcing this work to consultants led to higher costs and lower quality deliverables compared to products being released by tech companies and thus wanted to try a different approach.