Most of them are too stupid to come up with big, elaborate schemes, and the ones that try to come up with big, elaborate schemes are usually too stupid to cover up their tracks, and the ones who cover up their tracks in forming these big, elaborate schemes are usually still impeded by regular security measures. Then there are some that slip through, like McVeigh, the Crocus Hall terrorists, or Breivik. In cases like Crocus Hall, sometimes a big, elaborate scheme isn't even needed when security is simply lacking.