There's nothing preventing them from having different sized nets for different levels. Basketball 3-pt lines are different dimensions per level, for instance.
Kids are also often playing with 1 goal and 1 goalkeeper and 2 teams trying to score (you have to move past half the pitch after regaining possession before shooting)- that way on 1 field you can have 2 games going on at once and only 1 person has to be the goalie in each of these games.
And we've played soccer on random backyards - you just put 2 backpacks as the goalposts and you're ready to go. I think the focus on rules and dimensions is very american, kids just want to dribble and shoot, the score and the minute details aren't important :).
As for replay they were introduced in high-level football last few years. It's called VAR and most fans hate it because it spoils the celebrations after a goal is scored and you have to wait to know if it's really a goal or not.
> handball calls are still shambolic.
that's because handball calls are interpretation of intent and slow motion won't help you with that.
So then they changed the rules to ignore the intent and it became a valid strategy for a striker to try to shoot the ball at a defender arm for penalty.