[d]rnbqk2r/pp2npbp/3p2p1/2pPp3/2P1P3/2NB4/PP3PPP/R1BQK1NR w KQkq - 0 7
and, you migth want to analyse above position for as long as you want, I do not know how engines evaluate it, but, above only black has, sligth, but clear-cut advantage.
Why play 1.d5 only to get worse position with white?
Usual KID on 1.d4 with 1...Nf6 gives full equality or better play for black in most variations, though the single white defensive setup with f3,and possibly g4 and castling long away from the chain gives white some edge, not sure how big.
The f3 setup I have seen only in the games of Spassky and Petrosyan, but it is very strong indeed and what really should question mainline KID with 1...Nf6.
After 1.d4 c5, however, in the I-KID (
), black is simply better in all lines, if white does not capture on the second move 2.dc5, which gives full equality.That is why 1.d4 is, if not weak, certainly not the best possible move.
Would be very happy if someone manages to find a flaw in above analysis, or proves the position I posted does not favour black, but I think you will have very hard time at it.
PS. On a side note, it makes very much sense that 1.e4 is stronger than 1.d4. As kings usually castle short, the e4 pawn is closer to the king side than the d4 pawn, where it can attack, support other own pawns, etc.
While pieces move around easily, pawns are more or less a constant feature of the position, so it is very important where you got a pawn, closer to the king side or not.