Technically, all referees (and some coaches) aren't amateurs since they're being paid...but I won't get pedantic
Why wouldn't coaches be motivated to yell? They've invested lots of time and effort into their players (hopefully), and that naturally leads to feeling like they themselves have been wronged if they disagree with a call/non-call. I've never seen parents telling coaches to shut up, it wouldn't surprise me if it happened but in my experience it almost always goes the other way with coaches telling parents to chill out so they don't get bench minors (which btw I think is somewhat nonsensical that people in the stands that aren't on team rosters can cause teams to get bench minors, but I won't go down that road).
As for the difficulty of the job, this feels a lot like how the dynamics of police accountability is changing nowadays. You've got a lot of people who argue that police are human beings too and it's just as possible for them to make a mistake compared to a layman. I don't think this is a false argument, but I'm with the crowd that police officers should still be held to a higher standard and when they make mistakes, the consequences should be larger than for a civilian. After all, cops are being paid to do their jobs and enjoy privileges/rights that come with the position. If you can't cut it, you shouldn't be a cop.
Back to refereeing, I think some of this might be remedied if leagues made refs officiate a more consistent age/level group. It's kind of crazy refs can be calling a minor atom A game and throughout the course of the evening be calling a midget AAA game. Ideally refs should just be able to reset and officiate accordingly, but let's be honest it's not that simple. Even within the same age group, going from a A game to a AA game is noticeably faster and there's way more going on. So maybe they should create groupings so refs only work 1 minor/major age group +/-1 level for a month or something like that (ex. Minor + Major bantam, A & AA for a given month, or Minor + Major Midget, AA & AAA for another given month).