When we plant crops, we will inevitably displace and possibly kill sentient animals when we plant vegetables. Surely, however, there is a significant difference between raising and killing animals for food and unintentionally doing them harm in the course of planting vegetables, an activity that is itself intended to prevent the killing of sentient beings.
In order to understand this point, consider the following example. We build roads. We allow people to drive automobiles. We know as a statistical matter that when we build a road, some humans–we do not know who they are beforehand–will be harmed as the result of automobile accidents. Yet there is a fundamental moral difference between activity that has human harm as an inevitable but unintended consequence and the intentional killing of particular humans. Similarly, the fact that animals may be harmed as an unintended consequence of planting vegetables, even if we do not use toxic chemicals and even if we exercise great care to avoid harming animals, does not mean that it is morally acceptable to kill animals intentionally.
Moreover, because it takes so many pounds of plants to produce one pound of animal protein, we actually kill more animals in crop production when we are feeding those crops to animals rather than consuming the crops directly. So a vegan diet results in many fewer of those unintended and incidental deaths.
A related question is: why don’t plants have rights given that they are alive? This is the question that every vegetarian gets in the company of a nonvegan. These nonvegans may be otherwise rational and intelligent beings, but when confronted with a vegetarian, their discomfort with their diet often rises to the surface in the form of defensiveness.
No one really thinks that plants are the same as sentient nonhumans. If I ate your tomato and your dog, you would not regard those as similar acts. As far as we know, plants are not sentient. They are not conscious and able to experience pain. Plants do not have central nervous systems, endorphins, receptors for benzodiazepines, or any of the other indicia of sentience. Plants do not have interests; animals do.