Why are the doors locked after completing all quests and kill the last boss, do I then have to run all area to smash those things to open it again everytime ill go there, thats a bit stupid tbh…
And you dont have the quest for it so its hard to remember where they are those things to smash.
What doors? 
I think he means the door in the Fleshworks and the one in Crown Hill. They are for specific quests and you need to have the quest active for them to be opened.
Reach max rep with the new allied factions
The quest text will tell you where to go so no, it’s not that hard to “remember”.
So yeah, grind the reps, get the quest, and then go there and the door should be openable.