Because you would lose your grip on the center very quickly and be left with weak and/or isolated pawns.
3...c5 4. d4xc5 Bxe5
3...d6 4. e5xd6 Qxd6
3...d6 4. Nf3 d6xe5 5.Nxe5 Nd7 6. Bf4 Nxe5 7. Bxe5 Bxe5 8.d4xe5 Qxd1+ and you've lost castling privledges.
I constantly have similar questions about openings, "Why not this move???" and even though many seem "natural" (as some books put it) there is an inherent weakness. Typically, if a move is not in the top 3 or 4...it might lead to a weakening. Try playing out your variations on a chess program before trying any ideas online or over the board. Good luck.
why not play 3.e5 after 1.e4 g6 2.d4 Bg7 , controlling the center?