Honestly, because I'm worried that if I wasn't at least toned slightly then I would have a body unattractive to the opposite sex. Every little helps, you can't really do (much) about your face, why not make yourself attractive in ways that you can influence?
Coz one day I realised that having a BMI of 18 was not cool. 6'2" and just naturally skinny no matter what I ate. BMI is a rubbish indicator and whatever but still, I'm up to 21 now and heading for 23 ideally.