Gubment contractors, properly used, save the taxpayers money because they compete for the work and in order to win the contracts must cut overhead and fringe rates well below what it costs the government to employ a civil servant. Also, if a contractor employee is not performing satisfactorily it is far easier to have that person removed from the contract than it is to fire a civil servant. On my team of ten cyber security experts no fewer than five individuals have been removed for subpar performance over the past two plus years.
Contractors also offer a pool of specially skilled labor that can be hired for only the period they are needed, thus preventing the government from having to hire full time civil servants to perform duties only required on occasion. Now if the government "COR", or Contracting Officer's Representative, or "TPOC", or Technical Point of Contact don't properly perform their duties, it opens up the possibility that the contractor might take advantage of the situation. Does that happen? Unfortunately yes, but not to the extent you might think...once again, from the DoD perspective.