Ruy Lopez Berlin Defense/Wall: Why no Nf6?

Hello and thanks for your input!

After 1. e4 e5 2. Nf3 Nc6 3. Bb5 Nf6 (Starting the Berlin Wall) 4. O-O Nxe4 5. d4,

     "Nd6: 2,240

Be7: 609

a6: 609

exd4: 4" according to chess.com's opening masters database.

Instead of moving the knight to d6, why not simply move it back to f6? I've played this with people in real life twice now and that was their move. The database has literally no games of players moving back to f6. Why?

D6 ATTACKS BB5 FORCING BXC6 DXC6

AFTER NF6 POSTION LOOKS VERY BAD AFTER DXE5

It's always 5. .. Nd6 and never 5. ... Nf6. Why not?

because of 6.dxe5 Ne4 (back again =wasted a tempo)

The knight is slightly better placed on d6, since it's not blocking the d-pawn (white takes on c6, and black recaptures with his d-pawn) 

After 6: dxe5 Ne4 7: Qd5! White has a strong initiative, not what black wanted in his Berlin Wall. It could end like: 7: ... Nc5 8: Nc3! Just developing. Black has problems with his knight. 8: ... Be7 9: Be3 Ne6 10: Qe4 with a strong bind.