Answers:
1. this is basically opening theory in this example. The pawn march 5.e5 is too early. e5 is easily attacked with d7-d6 and an exchange makes your pawn structure worse.
2. technically, you don't have a pawn majority in the centre. Black has the same pawns you have. You have here more space in the centre. But space alone is not enough. Which squares are controlled by your pawns and pieces? Remember, the squares your pawns and pieces stand on are NOT controlled by you, unless they are covered by other piece(s) and/or pawn(s).
I've played a few games on this site now and one of the things that's been coming back every time is this: when I manage to gain a strong pawn presence in the center in the opening, I feel this should be an advantage for me. But my opponent then just advances and trades off a few of his own pawns, resulting in a stronger center for him. Why is it that I cannot hold on to my stronger center and exploit its advantage, that it is so easy for my opponent to undo my hard work?
For example here is one game where I gain an e5+d4 center as White.