No humans allowed

Robot Masters have AIs vastly superior to the smaller, minor robots within the Mega Man universe. This, even more than their generally humanoid appearance (Sniper Joes are also humanoid in appearance, yet obviously inferior to Robot Masters) is what separates Robot Masters from other advanced robots. They often are designed with a particular strength or element in mind, thus allowing them to thrive in specific environments. Robot Masters tend to be programmed with human-like personalities and quirks, advanced enough to fool most people into believing them to think and act of their own accord.