I haven't worked in game dev in decades, but I remember that just network synchronization could be tricky because the degree to which a door is open could lead to very different outcomes in a game. Eg take a shooter game:
Does the door block/deflect bullets?
Yes? Well now you need to make sure to synchronize clients really cleverly because each player will see a different state of the door depending on their latency. Whether they can shoot through the opening is relative to each clients' reference frame.
Oh, just make the server decide you say? Well, now all your inputs have a perceptible delay because your client has to wait for confirmation from the server, or you'll often see jarring prediction error correction when the thing you predicted is wrong.
A huge amount of research and work has gone into this and yet there's still no silver bullet.
That you think this is all solved just says that you know very little of the problem area - which is ok - but then you should not be making broad and confident statements about it. A touch of humility doesn't hurt.