As there are relatively few goals, anything that can turn a goal into not a goal or vice versa can have a massive impact on the game. For example the penalty decision against Croatia in the final.
Another thing that adds to the randomness is the chance that a key player may be sent off or injured.
To make that be specific -- in 44 of 64 games, and in every single penalty shoot-out, turning one goal into not a goal or vice-versa would've changed the outcome.
It is not so simple because goals in football are not independent from each other. A team that scores first has the opportunity to play more cautiously and go for more counter attacks.
Can't you put probabilities on that though? What you said is basically "impossible to predict, because one or the other might score more goals, so it's like coin flipping."
You can put probabilities on it, but the chance a game ends tied is rather high to begin with. 25% of this years knockout games were draws, and a 1/4 chance the game is decided by a coin flip is already enough to ruin predictions.
It's an incredibly low scoring sport with a single-elimination bracket. A fluke goal can swing the whole bracket.
In the NBA, NHL, or MLB, seven game series tend to even out the variance, so the best team usually wins. And even in NCAA basketball, there's enough scoring that any individual play loses significance.