taking back knight move

I have heard it said that a counter like nf6 to d4 is superior to d5 because you can take a knight move  back but not a pawn move. However, I have never seen in my games or any game where the knight has been moved an taken back to its original position after moving it. This phenomenon is rarer than the drunken knight opening. So I have 2 questions: why is this stated as an advantage when taking the knight back never happens? Also, why do people never bring a knight back to its original position? I have seen bishops sometimes brought back to their original location, but never the knight.

Nf6 is not "superior" to d5 and d5 is not "superior" to Nf6. There's variations that involve a knight retreating to it's original square. The breyer variation of the Ruy Lopez is one example.

Fressinet vs Carlsen blitz went 1.e4 Nf6 2.e5 Ng8 cue smirks from both players. Carlsen won I think, the cheeky swine. I never heard this particular reasoning for Nf6 vs d5 to be fair but there are some lines with knight retreats eg. Breyer version of Ruy Lopez.

I think you forgot to check the Zaire defense or the Monetevideo retreat. Of course 1.d4 d5 is not worse than 1.d4 Nf6. It is true you should be careful with pawn moves, but this doesn't apply to a perfectly sound and historical opening move.