Because you would have to measure every section of your body to ultimately say you're not losing inches.
I lose weight in my feet and hands first (!) -- my waist is the last place I lose weight. I tend to lose faster in my upper body, less fast around my thighs and hips.
So, if I am only measure my waist or hips, it would seem like I'm not losing inches at all, but I am losing weight. Or vice versa -- I can "gain" weight and it doesn't affect my size.
Sadly, we lose weight where our body wants us to lose weight first, not where we WANT to lose weight. Other people lose weight in the chest, first, for example! Thankfully, I don't.
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