What's wrong with 1. e4 c5 2. Nf6 d6 3. d4 cxd4  4. Nxd4 d5

I suppose a better question is, why have I never seen it. White loses the tempo, and black gains control of the middle. Is it because after the knight moves, white can play f4, and trade for the e pawn? Of course black can easily counter this with f6 and then it's basically just a trade of f pawns. What's the reason?

5 bb5+ knight go to f5 or trade bishops or on nc6 wins a pawn and exchange

no  ingles

hola

ni  llamo  es  kaesha

I'm also confused, d5 or e5?  Assuming e5, the move is not unknown but the more likely continuation (rather than the one you gave) is:
5. Bb5+ Nd7 6. Nf5 a6 7. Bxd7+ Qxd7 8. Nc3...

I see White with superior development, space, pawn structure.  I see Black with holes at d5 & b6, a misplaced Queen, jammed up pieces.  Black isn't lost but the chances seem to be more with White assuming he doesn't overreach.

sorry, its 4...e5. why isn't f4 a good move? Thanks for the feedback!

f4 serves no purpose for white. Weakens the Kingside & e pawn, doesn't aid development, causes the knight to be misplaced hinders the Queen bishop, gives black a target, usw.  Why would you play it?