Because most people's empathy doesn't extend to trees. That is the simple factual answer. All these other games about what trait draws the ethical line are post

Because most people's empathy doesn't extend to trees. That is the simple factual answer. All these other games about what trait draws the ethical line are post hoc. The reality is people feel it is bad to hurt animals they can empathize with. Though, to be fair, i don't personally think ethics can work any other way. We don't have slaves not because it is an objective crime against the gods, but because culturally it is wrong. Until your computer becomes sentient that is, then we will have slaves for another few centuries.