Becouse all people go to games explorer and look what move is most popular and play that move. So that move get more and more "votes"
But in this position, it opens c2 so you can place your Bishop there in case black decides to pawn rush you on the queen side, and also prepares d4.
Nc3 is a perfectly playable move. Keep in mind development is a means to an end not an end in itself. One reason grandmasters might like c3 so much is that it leads to such dynamic positions while Nc3 seems to lead to trades and maybe what grandmasters would consider an equal or dull position.
The idea behind c3 is white will strike in the center with d4. Black likes to expand on the queenside here e.g. with c5. So an additional benefit of c3 is to preserve the bishop pair. When black moves Na5 white has c2 for his bishop. This is another minor detail important to very strong players that don't make a tangible difference in (most) amateur games.
And because top players play c3 so much, everyone else tends to follow along :)
The idea of Nc3 would likely be to play Nd5 with some trades.
http://www.chess.com/explorer/index.html?id=4984&ply=14&black=0
I was using game explorer, and got to the above point. Why is c3 by far the most popular move? I'm in the middle of a big learning curve, but I was under the impression it is good to get one's knights in play early, and to center squares so you have many options from there. Yet Nc3 is a very unpopular choice here, as the percentages show. any thoughts on why? thanks!