Maybe they do have trials, we are just not showed the process?
Most superhero stories, at least in the early day, are very clear-cut in terms of the good guys and bad guys. So naturally the stories assume the bad guys are guilty no matter what, so it would be rather redundant to show the entire process of a trial when you know they will be found guilty anyway. They are just focusing on the more interesting stuff.
I asked a question here years ago about why couples in film-noir fell in love just after days of meeting each other. I remember one answer said that they might have been around each other for much longer than I’m showed. I think this follows the same logic here.