Your trip quick to google didn't serve your argument.

In today's usage, the verb form 'perfects' sadly/inaccurately carries a 'pluperfect' meaning (i.e., having been completed already), and as such that older expression doesn't 'serve' the actual meaning of the marvelously rich idea of "grace perfects nature."

So most current uses of this idea (even among school trained theologians, that is not you and me) concerning the very big idea of "what God is trying to do with our nature" use this version instead: "grace builds on nature".

Americans would otherwise tend to think their cooperation is not needed since God has (in a pluperfect sense) has already completed the work!

The idea, after all, should serve a "pastoral purpose", that is, our steady and never ending growth in holiness.