The real reason is that people make mistakes all the time. There is no shortage of potential mass murderers, are there are plenty of successful ones. But if the

The real reason is that people make mistakes all the time. There is no shortage of potential mass murderers, are there are plenty of successful ones. But if their plans are too ambitious or involve too many people, they tend to fail due to stupid mistakes. And when those stupid mistakes happen, security agencies (and even ordinary police) have a good chance of catching them.