Because body positivity is not about looking good, it's about feeling good. Because when deluded fatties lose weight, it's incidental, or for health purposes, not for looking good, because that would imply being fat is less attractive. Anathema. Because when they build muscles, it's for strength or for health purposes, not to look good, because that would imply being a formless, toneless blob is less attractive. Don't question it, just accept. You're dealing with fatlogic, it will never make sense.