Because the fighter is the one who specialises in taking advantage in small gaps in the opponent's guard. Take your eyes off the barbarian and he will wind up for another swing. Take your eyes off the fighter and he'll try to slip a sword point through your defences in the same amount of time.
You should only not be more afraid of taking your attention off the fighter if (a) you are immune to being stabbed (in which case you shouldn't fear the barbarian either), (b) you are immune to fear (in which case who cares?) or (c) you suck at threat assessment.
You can't intimidate a golem. But you can stab them. If the golem attacks the barbarian the fighter is likely to stab them, knocking them off balance in the process. Golems can ignore the fear effect. They can not ignore the stabbed and pushed off balance effect.
Or you can work out what they actually do. And that effects like Marking and Combat Challenge (the two go together for a fighter) would be entirely at home in a grittily realistic game. 4e is (like all D&D) a game that runs on what can charitably called action movie physics
Or is it impossible in your universe to (a) get in people's faces and knock them off balance or (b) be better than other characters at taking advantage of minor lapses in concentration?
Then for the love of Gygax stop demonstrating that you just don't understand how these things work to the point that you think that people only get distracted by people who test their guard and physically bully them if you are using action movie physics.
We have different thoughts and opinions on this. I played 4E for it's life cycle I understand how it works. I thought many of the 4E fighter powers were supernatural. Feel free to differ.