1) In all honesty, it’s just general lazyness which we all can relate to. It is easier to sit and wait while a few others go and complete the pre-events, and I doubt anything will change that until Anet are able to incentivize the pre-events significantly more (such as the lootbags earned from the Modniir pre-events).
2) It is the players themselves that make encounters so; eg players discover safe spots to stand in, which developers likely wouldn’t have intended most players to exploit (Golem Mark II). Of course, there are some encounters which are very dynamic and fast-paced; Tequatl and Triple Trouble Wurm to name a couple, which I find are a blast to play even to this day. It also seems that future bosses are becoming much more dynamic and flexible, as seen from Season 2 of the Living World’s instance bosses, and the footage seen of the wyvern fight in Heart of Thorns.
Do note that all of this is solely my opinion and is very likely to differ to others’.
If you are around other players, you will be sharing buffs and ressing the downed before they die completely, etc. That’s why we stack up.
There aren’t really any safe spots for any of the world bosses…. but some areas take quite a beating, while others are barely touched.
Take the claw of jormag for example… everyone stacks at the safe distance from the boss (where you aren’t taking chill damage) and ranges the wall down.
Standing any closer will just get you killed….. guaranteed.
Sticking with other players ensures that you will be sharing stability buffs while simultaneously ensuring there aren’t players running around dragging upscaled champions into the players who just want to fight the boss.
You can also easily resurrect any players who might get downed.
…and the reason you stand on the ledge is so you can watch the ledges, which is quite possibly the most important part of the entire fight.
