The same reason anaesthesia doesn’t make it okay to kill humans. There’s lots more on this in chapter 4 (“What's Wrong with Killing?”) of Singer’s Practical Ethics, if you’re interested.
Well, if we are to follow the parent's mention of Peter Singer, you could go check out just about any source of preference utilitarianism. Briefly:
An animal has a conscious will to live on, a preference so to say, and those preferences are the basis of moral consideration.
A plant lacks those kinds of preferences (lacking any brain activity - what we can see are mostly just hormonal cell responses and such; things that all living cells have) and thus the plant in itself is not worthy of moral consideration.