The reason people rewatch fights is that for the average fan, regardless of whether they're casual or hardcore, an in-the-moment decision is rarely unbiased. On fight night, you've got who you want to win, you've got the hype, you've got the commentators saying whatever they want, and all that makes it hard to be objective.
For the judges, though, that's their profession - to be completely unbiased. I'm not sure why you think a fan watches way more fights than a judge, because there's no way it's true. They are literally paid to be at fights, staring intensely at the action with an impartial eye. The average fan probably skips a card or two, probably watches the prelims in the background, and probably only focuses on two to four of the main card fights. Even a hardcore fan is very unlikely to watch more MMA than a professional judge.
So, to answer your questions in brief, an in-the-moment decision from a fan means very little, and the judges are more often than not going to get the call right.
