Maybe because sometimes it can be relatively easy to to write some software, but much harder to find its bugs and write exploits for them? otherwise, why would bugs even exist?
people make mistakes, it is innate feature of human being. Bugs exist because it is not economically effective to find them. Bugs, given unlimited amount of resources, is possible easy to find whenever they are. Google how one guy hacked infamous HackingTeam alone. He provided some estimates how long it took him. Does it make his software the most sophisticated one on the planet? If his software is the most sophisticated, can he develop Windows alone?