that's because people think calipers work. They do but you have to multiply with 2. Works most of the time.
Actually, calipers can work well if one uses them correctly (Jackson & Pollock 3 site = pretty good). The problem is that most people fucks up the measurement or intentionally fails to grab all the skin that's there, and they do not account for fat other places in the body.
If anything, if used properly with a tracking program, calipers = good to use for assessing progress.