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 ...
What is a so called 'best' move ??
Are moves played by deep engines equivalent to real moves by real people with real limitations and genuine concerns really the same?
Who is to say what is 'best' when two people play.
I recently had a game (10min) against a higher rated opponent.
He moved his queen and I took it. He resigned. What he did not see was that ...
Moves that are significantly worse than the best option are annotated with ?!. If the difference between the move in a game and the "best" move is not big enough, then it doesn't matter whether some engine gives one a 0.02 better evaluation than the other.
In some sense, you could argue that !, !! and no annotation at all already indicate that the ...
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 ...
(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.
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 ...
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.
Not many moves in the opening deserve exclams. A classic example was Morphy-Allies, Paris 1858: 1. e4 e5 2. Nf3 d6 3. d4 Bg4 4. dxe5! and Black will lose a pawn, or lose bishop for knight while enabling White to gain time by developing with the recapture.