A rough rule of thumb in military circles is that in kinetic warfare an attacker needs a three to one advantage in manpower and firepower in order to successfully defeat a defender. Defenders typically have an advantage because it is normally easier to protect and hold than it is to move forwards, to destroy and to take. However, in cyber warfare the opposite is true. Attackers have an enormous advantage, maybe by a factor of as much as 10 to one. Large institutions must defend against many thousands of attacks every day. Only one needs to get through for an attacker to succeed. Generally speaking, offensive cyber-attacks are low cost with a high payoff, where defensive operations are expensive, overstretched and often ineffective.
Cyber warfare is asymmetric. Both the attacker and the defender are in a race to find vulnerabilities. But the number of vulnerabilities grows exponentially with the size and complexity of the system. The defender has little chance of finding every single vulnerability and patching it before the attacker finds one to exploit.