Because its the easiest looksmax to do and you won't really know how you would look like unless you lose weight. Look at incelselfie and see the amount of literal obese fat fucks there that have average eye area and noses.
The only way you are incel or truecel as a fatcel is that you're either, a shit nosecel, an eye area cel or both as they can't be fixed by losing fat. (guess who has both) so just by losing your body fat to about 20% you'd already know if it's over or not. That's why people love saying fatcel is volcel.