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.
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.