after 1 d4 Nf6 2 c4 g6 3 Nc3 Bg7 4 e4 d6 5 Nf3 O-O

  what's wrong with 6 e5?

Well, 6...dxe5 asks a tricky question :

a) if 7.dxe5?! Qxd1+ 8.Nxd1 Ng4 and the e5 pawn looks difficult to defend

b) if 7.Nxe5 Nbd7 and Black looks comfortable.

With 6.e5, white is opening the position while he is less developed than black, so it's maybe dangerous