Mercenaries is not the preferred term. Contractors is--since it does not carry such intensely negative connotations--unless we speak of contract killing. Contractors accurately describes the diverse group of civilians and military reservists who provide services during America's conflicts. The U.S. has been using contractors throughout its history; for example, there were many during the Civil War. Using contractors or being a contractor is not bad per se-- take for example a Just War. Contractors tend to work for relatively short periods of time, generally one or two years. Many of them have everyday jobs, and many are not American. Using local contractors can be a good way to earn the loyalty of the local population by creating jobs.
One might see a gate guard from Albania in Iraq or a cook from Sri Lanka in Afghanistan. Both are being paid by Uncle Sam, and both are contractors. Both will generally work for one or two years. But most of the cooks and laborers, for example, will be local. The companies that hire them are sometimes domestic.
Contractors who conduct war in the battlespace, who might be termed mercenaries, are used by the United States for many complex reasons:
WELCOME TO REALITY: DON"T WORRY, YOU'LL BE BACK SOON
It is very difficult to foresee the numbers of people and the skill sets that will be needed in a conflict, no matter how likely such a conflict might be and how well one knows the human terrain. That certainly was the case during the lead-up to the second invasion of Iraq. People with special skills can come in as contractors very quickly, and they do not need to go through the long process of military training. It is not that they generally tend to be civilians. In point of fact, many contractors were military reservists. The United States can go into a country without declaring war anyway. It does it all the time.
Everyone likes being a contractor because the money is so copious. The happiest people are those who supply contractors and made sure that the U.S. keeps using them. Northrup-Grumman is a good example. American-citizen ontractors make a lot of money, but the people who manage the programs and own the companies make more, even incredible mountains of loot. It's a cash bonanza, a permit to print. Everyone involved benefits greatly.
The wars can continue longer, which is another way of saying that the U.S. can maintain a presence longer, because not as many soldiers are dying. Contractor deaths are not as visible, and contractors usually have a smaller footprint and cause less heartache to the locals.
Yes, the presence of contractors can be denied, one imagines, but how often is that really needed?
In short, it is all about money. The uncomfortable truth is that the United States was attacked on 9-11 and anything that resembled an intelligent response was overtaken and determined, at least to a significant degree, by people who wanted to get rich--it was like the Wild Wild West around Crystal City in Arlington. The war was mostly paid for by debt. It became vendor driven. The United States then prosecuted war based upon what was really individual self-interest, and that meant getting as much money as fast as one could no matter what. Doing the less expensive thing that might work was often taken off the table. Not interested. And this accounts for the length of the U.S. involvement in several conflict zones. Nobody wants it to stop, and remember that being in a combat zone means that you will not have to pay taxes for that month.
Doing the most expensive thing possible by using contractors was the method of choice in a de-centralized scheme in which hyper-self-interested bureaucracies were allowed to compete for turf and budget. If the money is not spent, then the organization is not important--and your people will not get that money next fiscal year. Some say it goes like this: the enemy leadership, let's take the example the Senior Taliban Leadership, had no business sticking their little noses in the fight between U.S. entity X and U.S. entity Y. Afghanistan was a stage set and the locals were props in a drama between titans. Contractors were important characters in the story, folks who got things done, at least in the short term, without ending the cash bonanza. So here is another reason to use contractors, which, I am sorry to say, rings true: they will prolong the gravy train without winning the war and ending the champagne sparkle in everyone's eyes back in D.C.
Congress wrote a blank check, and that spawned run-away contractor spending. Such flows of money to the private sector under the table and on the table were made convenient by the insularity and buddy systems of Washington D.C. itself. The friends of the friends got wealthy through using contractors instead of using the military for everything, active or reserve. It was all done right in front of everyone's faces. It's all in the news. Contracts were done without competition for projects that never got finished.
On 9-11 the U.S. had a large pool of people with security clearances who were not in government service anymore, and those people were therefore very valuable. They could step into important jobs as contractors because they had already been cleared.
There are some folks who are valuable to have in war, but they are too old or otherwise unable to join the military. They can be brought in as contractors.