It's not illegal, I think more likely is each Racetam has their own unique effects and strengths, so stacks are rare because people tend to like to try them by themselves to asses the effects, and also minimise side effects. For example some people can get headaches from Piracetam, but do not from Aniracetam. It's a good question though! And I admit I don't know why exactly, I would have thought there would be more stacks. Although having said that, I do know back about 5 years ago when Amazon was selling Piracetam and other Racetams (before removing them all from their site), there was a stack with Aniracetam, and Oxiracetam, and perhaps one other but I can't recall exactly. Do let us know if you find any!