Why does everyone play e4 or d4?

I always feel a little nervous moving my center pawns forward two squares on the first move, so I like playing b3 or g3 on the first move and then I move my bishop into the little hole there. I heard some people calling this a fianchetto. What is the name of this opening? And I think people should try to surprise their opponents more in the opening phase, it often leads to more exciting games :-)

at the pro level if your opponent spent a lot of time preparing against a certain opening then you can totally psychologically get the advantage by playing something else.

Sorry, but I find b3 or g3 less exciting than 1e4 and from my limited understanding of 1d4, b3 and g3 are less exciting than that.

I am nervous *not* moving one of my central pawns forward two squares. They are such strong moves it is hard not to play them. 

I play e4 and, if I can, d4. This controls the center squares and enables my bishops to get out. It's a no brainer. 

e4 - fast development of king bishoop enables fast 0-0 what means that white can plan his game and attack if needed, verly early

A lot of pawn structures can occur when starting with 1.g3/g6 that are very complicated. For instance, the Saemisch pawn structure (c4/d4/e4/f3) is a very complicated game. Or what if they play f3 but omit c4? What if they play c3/d4/e4/f4? Or c4/d4/e4/f4? (Flip notations for Black.) My point is that it is very difficult to grasp an understanding of all these pawn structures. I can't imagine trying to teach someone how to play all these structures, nor could I recommend playing cramped positions. Furthermore, preparation for playing 1.g3 is difficult too. The transpositions that can occur are vast. The reversed Pirc, reversed KID, etc., lead to complex positions. On the other hand, 1...g6 is very difficult to play vs the Averbakh structure (c4/d4/e4), especially if f3 is played too. I could go on and on, as someone who tried to play a 1.g3 and 1...g6 repertoire! But, I'm still sleepy.