Writing Fat Characters

Why are we ”fat shaming” our fictional characters? Is the demand of being skinny and successful really so strong that we shy away from writing fat characters that are not just the bullied supporting character? Is there really not a market for the fat (but not always the funny) character that can find love, that in turn is not hopeless?
Is the power of our fat shaming society so strong that it seeps into the imagination of our writers??