Why no collision detection between PCs and (N)PCs ?
The first and obvious answer is because some guys may abuse of it voluntary or not.
I have played once Tibia and there were \"thiefs\" using the collion detection eg \"Give me something or I (and my friends) won\'t let you pass\".
Another example, a player is afk or having network problems, the character won\'t react. So It could bloc critical access.
The last one, you are in town so there is a lot of people. You want to go to the other extremety of the street, but other characters are walking with the opposite direction. How long will you stay in game before disconnecting?
I guess collision detection will also need more CPU, I don\'t how much, but it could increase lag...
(Bonus Question : How many people would be needed to bloc the city gates?)
PS : Of course there is gm, but that is not a sufficiant reason to create additional problems.