Many of the Guantanamo prisoners, if released, have no place to go. No country will accept them. Do you turn them loose in the United States? These guys would love that. Even if you put them under 24/7 surveillance (very expensive), the tail is simply seen as a challenge to the terrorists, who already know how easy it is to disappear in the United States. Even when you are being watched.
Even if there is available evidence to convict a terrorist in a conventional court, the rules-of-evidence (which allow the accused to scrutinize the evidence) puts the sources and methods of that evidence at risk of becoming known to the terrorists. This makes it easier for other terrorists to evade capture, and enables them to identify and kill those who assisted in the capture of terrorists being tried in the United States.
This process also brings politicians and lawyers into the intelligence collection process, something that makes counter-terror efforts more difficult. For example, as with police investigations of criminals, collecting intelligence on terrorists often means dealing with some unsavory people. This is political dynamite, because the mass media likes to portray this as somehow unethical. This is never an issue when common criminals are sought as sources of evidence, or even witnesses to crimes. But when terrorism is involved, it becomes an issue. Government bureaucrats tend to respond by installing more lawyers to supervise the intelligence collection effort. This, in turn, persuades the intelligence bureaucrats to order their field operatives to simply stay away from any source that might later prove embarrassing. That results in much less intelligence, fewer captured terrorists, and more terrorist attacks. If that leads to a large loss of American life, the restrictions are quietly lifted, and the field operatives are instructed to go Jack Bauer on the bad guys until the problem is dealt with. Then the cycle begins again.