Well, there's Tool's "Disgustipated", about the carrot harvest. And some people do think that plants can suffer. Indeed, some Buddhists only eat food that would otherwise be thrown away.
But yes, the relevance of suffering seems proportional to relatedness with the sufferer. Or maybe the ability to identify with the sufferer, based on perceived intelligence and self-awareness.
But we do have a lot of reasons to believe that animals can and do suffer, but no, or very few, reasons to believe that carrots can.
It's also not so black or white. Does a cow have greater "capacity for suffering" than an oyster? Does this have ethical consequences? I think it does.
And yes, maybe oysters are closer to plants than cows, in that regard.
But it's not just about suffering. It's about the morality of killing other lifeforms. Personally, I'm OK with it. I'll be eaten after I die. Indeed, I'm always under attack from bacteria, yeast, molds, etc. But I won't eat anything that I wouldn't kill myself.
But yes, the relevance of suffering seems proportional to relatedness with the sufferer. Or maybe the ability to identify with the sufferer, based on perceived intelligence and self-awareness.