Why is 3. Nc3 played in mainline of the Modern Defense? After:

[FEN ""]

1. e4 g6 2. d4 Bg7


by far the most common third move is 3. Nc3. This goes against the principle of not blocking the c pawn with the knight as white, now of course principles can be broken but I can't see the benefit of doing so here.

The e4 pawn does not need defending, and if the aim is to stop 3... d5 then 3. c4 achieves this as well, then Nc3 can be played later.

So why is it not preferable to play 3. c4 before Nc3?
3. c4 will likely transpose to a King Indian defense while 3. Nc3 will likely lead to a Pirc. 1. e4 players will prefer the more familiar Pirc positions.

The Modern Defense move order lets White decide the main position.