Primarily because they're hard to take accurately. Using calipers takes training; the bathroom scales which report body fat percentage are using an approximation based on conductivity which have varying results. BMI is quick, and I agree not a very good measure, particularly if only used on their own. For example, lean, muscular people can have BMI values which, outside of context, place them as morbidly obese. But, because they're expedient, they've continued to be used.