There is no known way to guarantee that a system is secure. Vulnerabilities exist in hardware, software, and throughout the supply chain. They may exist by accident or by ignorance. They may have been inserted maliciously, or they may utilize some mechanism never before considered part of the attack plane. There is no one method by which all these potential issues can be addressed, and no tool that can find them all.
Security has often been likened to a castle. You build layers of protection and while you expect some to be breached, you make it increasingly difficult to get the ultimate prize. Verification has a role to play in exposing vulnerabilities, but the problem is that entrenched dynamic verification methodologies have difficulties with this. Technologies are needed that can produce testcases which demonstrate a weakness. Formal verification is one that has been successfully used for this.