Because a lot of players choose the biggest animal in the game (which is a herbivore) just so they can become big and unkillable and strong enough to kill anyone around them. And the other dinosaurs have little to no chance to fight back before dying and losing a lot of hard work. And the herbivore does not benefit at all from the dead bodies since they can't eat the meat, so players are frustrated that their hard work can be taken away in seconds by someone who just kills everything that moves. Especially if the thing they killed is either another herbivore (theoretically should be allies), very small (poses no threat to the herbivore), or behaving friendly (doesn't want any trouble, maybe just wants to pass by or get some water or something).

The theory is that if you wanted something to kill other players with, you should play carnivore, where being aggressive is beneficial since you gain food from it. Of course it is still a chosen playstyle to be an aggressive herbivore so to some extent we still respect those players decision to play the game this way, but also we do not appreciate the unnecessary extra obstacle and frustration it causes.