1. your legs occupy about fifty percent of your body. This fact alone tells you they need as much attention as the other half of your body. Do not forget your body is a bio feedback mechanism. So the more you train the better your body will respond. It will release more hormones in response to the training like testosterone and growth hormone and these hormones will go through your entire body not just in your legs. So by training just upper body you only get %50 of these.
2. Your legs are your foundation. Without a strong foundation you will crumble just like a building would. There are countless exercises that rely on your legs for support. So these people are actually holding up their progress in there training regiments by neglecting their legs.
3. Your physique should be proportional. You do not want to walk around looking like some strange cartoon character. With all these developed muscles in your upper body and these chicken legs it looks ridiculous. It reminds me of the S.N.L skit "how much ya bench". I have attached this to the post. Enjoy!