As the title above, why aren't early opening moves generally given exclamation marks? I see question marks for obvious opening mistakes from time to time, e.g. 1.e4 e5 2.Nf3 f6? or 1.d4 d5 2.Nf3 Bg4?! But I almost never see exclamation marks for early strong opening moves. For example, after 1.d4 d5 2.c4 should be given an exclamation mark, being obviously a strong move, already challenging Black's center at move 2, or, 1.e4 e5 2.Nf3 Nc6 3.Bb5! the Ruy Lopez being considered the best way to challenge Black's center already at move 3, and so on...
Wikipedia says the following:
Exclamation points ("!") indicate good moves—especially ones which are surprising or involve particular skill. Hence annotators are usually somewhat conservative with the use of this symbol.
(emphasis mine). 2. c4 is the most played move after 1. d4 d5, therefore it doesn't receive an exclamation mark.
To try to give a concrete example consider Sax-Seirawan, Brussels 1988
[Event "World Cup"] [Site "Brussels"] [Date "1988.??.??"] [Round "?"] [White "Sax, Gyula"] [Black "Seirawan, Yasser"] [Result "1/2-1/2"] [WhiteElo "2610"] [BlackElo "2595"] [ECO "B09"] [FEN ""] 1. e4 d6 2. d4 Nf6 3. Nc3 g6 4. f4 Bg7 5. Nf3 c5 6. Bb5+ Bd7 7. e5 Ng4 8. e6 fxe6! 9. Ng5 Bxb5 10. Nxe6 Bxd4!! 11. Nxd8 Bf2+ 12. Kd2 Be3+ 1/2-1/2
(Annotation is mine to make the point)
In 1988 before this game
8 ... fxe6 was considered a blunder and the whole
5 ... c5 line was considered somewhat dubious. However Seirawan's find of
10 ... Bxd4!! effectively gave birth to a major new line in the Pirc, and Chess Informant voted this the most theoretically important game for the first half of 1988. Later white found ways to avoid the draw, and line is considered fairly equal nowadays, but even back then such strong novelties as early as move 10 were rare, and now, especially with engines coming into use, have all but disappeared.
So back in 1988 Bxd4 was a very surprising, strong move in a position where Black previously had been thought all but dead. Thus back in 1988 it deserved at least one if not more exclams. However nowadays it is routine - as a ~1850 patzer I have had this very game a number of times. It has lost its surprise value, and so no longer deserves the annotation.
Exclamation (!) is a highlight in analysis. Either it indicates strong move, or it is also possible to indicate unexpected move. Something to which opponent is totally unprepared. Something which will be followed by inaccuracy (?). This could be different depending on what exact game it is. What will follow after provoking move? Exclamated move could be not optimal at all, making a weakness in your defense. Nonetheless, even making you weaker, it could lure your opponent into blundering entire game.
If that is the game from XIX century where opponents could be not prepared and preferred to accept gambits, then it was "!" move.
Today we have openings up to 12 moves, and the theory behind every variant shows equality, so even if it is Queen Gambit Accepted variant - opponents are already expecting you playing it, you would not catch any initiative.
It all depends what century is the game from.
Today everybody knows QGA and most likely will choose 1.d4 d5 2.c4 c6 and there will be no surprise in that.
As noted by the existing answers, opening moves are usually standard and as such expected to at least some degree.
Arguably, Fischer’s 1. c4 in the sixth game of the 1972 World Championship match could deserve an exclamation mark, since it was a remarkably good strategical move, albeit somewhat “meta-strategic”.
The !, ?, !?, and other such symbols are meant to highlight moves that are unusual in some way: Unusually good, unusually bad, unusually dubious depending on the opponent's response, etc.
If they were used on opening moves, then they'd appear with practically every single game of chess.
At that point they'd become just noise and people would ignore them. To have any impact, they have to be used rarely.
Lots of good answers here, but I'll add one perspective that's kind of been missed.
Every chess game starts out in a position that has been played before by others. The game continues in this state until, at some point, in some games, play evolves into a position that has never been played before. It is certain that you will not find an
! on moves that simply replicate a game that has already been played before because there is nothing surprising about copying a line from another player into a position that both players know, understand, and have seen before.
Only once a game moves into a novel position are the players truly challenged to demonstrate their skill and you will typically not see
! used until it produces such a novel position, and only rarely even then.
When Einstein published the theory of relativity, he was hailed as a genius.
When an undergraduate physics student turns in the same equations as homework a hundred years later, they simply get to come back the next day instead of being sent home with an F.
! is relative. If you're the first person to make that move, sure - you deserve it. If, 20 years later, that move is standard play into a line 3 moves deeper yet and you're just dancing through an opening with your opponent...well, sorry, no
! for you.
(Arguably, Go is a deeper game and in any event, don't you have something better to do with your time than play games?)
Symbols like ? and ! are not meant to communicate opinions, which is what they would be if you sprinkled them on standard opening moves.
To forestall an objection, an informed opinion is still an opinion.