Lack of Resources. They can take 1 or 2 targets, once in awhile. To cause irreparable harm would require many targets be hit all at once. Even then some of these targets could be recovered relatively quickly making any harm temporary. Many others only increase risk of harm and are not automatically harmful to lose by themselves.
Would you volunteer to staff the first site to be hit? How about the second one, if you survive the first attempt?
Would you commit to living a winter without heat in empathy to those who would freeze should a gas pipeline junction point be destroyed?
Would you be the first to volunteer to drink contaminated water in empathy with the families that will die as a result of a violent political statement?
How about if you volunteer to be infected with a fatal if not treated disease and then refuse treatment in empathy with those who will die if a vaccine lab is destroyed?
Or is it just that you, like your friends at WikiLeaks, see the casualties of any possible terrorist/revolutionary/anarchist attack as inconsequential collateral damage in furtherance of your goal of destroying the United States?