Because plants and fungi aren’t sentient and cannot feel pain. They’re alive, but not conscious. They don’t have wants, they don’t cower in fear when you hurt them, they don’t feel or think, they just are. Animals can think and have feelings, so the idea of killing one just to eat when I can eat other things that don’t. Not that I think I alone have some impact on the number of animals killed every year, but I just can’t stomach eating meat when I know something with a mind had to die for it