Because there are many small air gaps between the shoes and the feet. The impedance mismatch would be huge, and likely throw your signal deep into noise territory. Have you seen the amount of jelly they spread on your skin for medical ultrasounds? I don't think having to submerge your feet in liquid when trying out shoes would win many customers.
But in this case you just need a very rough 2d outline. I believe some stud finders use ultrasound. This might be more similar to that than a medical ultrasound.
