I was so frustrated with Fred during his job interview!

I think we have to consider the mental & emotional state Fred is in after returning home. Physically, he's home, even though it can't be as it was before—but psychologically, he's still back in the war, still carrying the brutal toll it took on him. His brief marriage is a sham & has come apart; he's filled with resentment that younger people are getting jobs & doing better than he is; he's got despair creeping after him as if it's his own shadow; he's wondering if the risks he took & the sacrifices he made for his country even matter to anyone any more—how can he talk himself up, if he doesn't even believe in himself any longer?

This happens a lot in life.

The one place in the world that you DO NOT WANT TO WORK, but are qualified to work and they offer you a job.

Also, we aren't used to seeing a classical alpha male in such a low demeaning role, even in real life. This type of person rarely has this type of job. Not a complaint, an observation.

He didn't expect to have that job interview. He goes into the store on impulse to say hello to his old boss, who pushes him into talking to the new boss about a job.

^ This. Also, the new owner was hardly "begging him to talk himself up". Pretty much the very first thing out of the guy's mouth when he meets Fred is that he's under no obligation to give Fred a job. He doesn't really want Fred there any more than Fred himself wants to be there; it's like Al's situation at the bank with the higher-ups there doing quite literally the bare minimum in terms of helping veterans but clearly not even wanting to do that much. The entire point is that Fred does not want to go back and work at the same thing he was doing and in the same place he was working before the war, but that's equally pretty much all he's actually qualified to do. Any skills he may have gained in the interim are only of use within the context of war, at least as far as Fred's concerned. In the air force, he was a decorated officer with several commendations for bravery and earning quite a lot of money for what was, to him, probably quite simple work; in the regular world that he returns to, he's just plain old Fred Derry who clearly was born and raised in relative poverty, who can only work what's probably a minimum-wage job and whom virtually no one (including his clearly expensive wife) really gives a toss about. It's just one more aspect of the social disconnect between returning soldiers and the changed world they come back to that the film explores.