Why is eating meat morally wrong?

I don't understand why vegans believe it is okay to kill plants, and microscopic organisms but not animals. Is it because animals can actively protest their slaughter by yelping and moving whereas plants just stay motionless. A plant doesn't wish to die. They perform' metabolic actions to preserve their lives when punctured. They may not yelp like an animal but that's because they can't. What fundamental distinction is applied when evaluating immoral killing and moral killing.