actually, i know why!
when sketching, your brain automatically focuses on the neatest line and ignores the rest. Without the other lines to focus on, your brain can see more 'wobbly parts' and thus looks worse.
https://www.deviantart.com/chimchamtimtam/art/ftu-cat-base-lineart-774287293
https://www.deviantart.com/charlie-mom/art/F2U-Plushie-Mega-Pack-Series-1--789704024
https://www.deviantart.com/iipaw/art/FTU-Warrior-Cat-Feline-Lineart-742617159
like this?