It might have something to do with the process of making a robot with a soul. She called herself a "combat android capable of projecting an Aura," but projecting an Aura requires a soul, and everyone in-universe knows that. On top of that, the four kingdoms clearly aren't as friendly with each other as they like to appear when speaking to the public. Penny was almost certainly the prototype for an espionage unit.

And on top of everything else, she has one specific "father". She may have been built to look human because that man wanted a daughter. From the glimpse we saw of probably-him, he's pretty old.