Well, we do and did, but you are on the right track. The words are τελειος, πληπης and τελειωμενος. All of these words imply either completeness or fullness or a state of being finished. These concepts don't really apply to God, but we do have to say something, don't we? :)
The English word "perfect" might be a utopian concept..
Complete or finished means something, perfect is nebulous..