I think we've invented theory , what do you think ?

Also , why does no-one play 2.Bc5 3.d4 in the Knight Variation  of the French Defence after 1. e4 e6 2. Nf3 ?

Because 2...Bc5 is bad as 3 d4 wins a tempo.

Then why is 3.d4 never played according to chess.com's explorer database ?!

#3
The games with 2...Bc5 are probably between weak players.

Only a bad player wouldn't play 3. d4.

According to the database 3.Qe2 3.d3 and 3.c4 have been played in that position but not 3.d4 , why is that ?

What if theory invented us? If it did, do you wonder if theory ever sits around posting questions on their Theory.com website?

#7
Look at the ratings of the players who played 2...Bc5?
Look at the ratings of the players who played 3 Qe2?, 3 d3? and 3 c4?.