Why aren't all chess games the same?

Ok so I'm a begginer in chess, so this question might seem kind of stupid but hear me out.

I suppose all grand masters or international masters have studied the best moves in chess a million times, and must have arrived at a conclusion... So if chess is something objective and there's a best move in each situation, why don't they always play the same exact game? I mean, if a game is different from the "ideal game" it means that one of them made a mistake, right?

I hope you understand what I mean...

@1

"if chess is something objective and there's a best move in each situation"
++ There are usually several good moves and several bad moves.
In the initial position 1 e4, 1 d4, 1 c4, 1 Nf3 are about equally good.
After 1 e4 the moves 1...e5, 1...c5 and maybe 1...e6, 1...c6 are about equally good.
After 1 d4 the moves 1...d5 and 1...Nf6 are equally good.
After 1 c4 the moves 1...e5, 1...c5, 1...Nf6 are about equally good.
After 1 Nf3 the moves 1...d5, 1...Nf6, 1...c5 are about equally good.
So just after move 1 there are already a dozen equally good continuations.

"if a game is different from the "ideal game" it means that one of them made a mistake, right?"
++ The ideal game is a draw. Each decisive game has an odd number of mistakes.