Plus a certain percentage of these people take the diverse base of experience and go start something useful. Teaching a bunch of smart, ambitious people about how the process actually works and then letting them go start their own operation seems like a recipe for job creation.... (and finding better ways to replace the status quo)
Also, in practice, contractors build a thing as fast as they can, then leave and never come back. Agencies that lean heavily on contractors generally don't have a deep bench of technical talent, so they get stuck with a maintenance nightmare that they didn't build and have a hard time understanding.
My feeling is that contractors tend to build differently than people who will have to support the system for however long the law it is enacting exists...
Trump has expressed a preference for private industry to do things for the government, because they do it better. Accenture, Deloitte and KPMG are prolly the federal IT of the future