Having spent 2 years of my life working on research projects, one measuring children aged around 7 years and their mothers, the other on adults in various studies, I can assure you that getting accurate measures of human bodies does require proper equipment and skilled operators. You can't measure yourself accurately; someone else really does have to ensure correct angle of head and read the height measure (nearly everyone gains 1 - 2 cm in height when measured correctly) and hold the tape measure in the right place. The waist measure used in human anthropometry isn't waistband of trousers or skirt, it's half way between top of iliac crest and bottom of ribs, which takes a bit of prodding to find, and is more difficult the chubbier the person being measures.
Body composition scales aren't cheap - several hundred pounds for a Tanita that you stand on, for example, and even more for one on which you hold the handles as well. A Leicester Height Measure (accurate, plastic, can be folded up to be carried around) costs over ?60. If a height rule is attached to a wall, it must be a vertical wall - you'd be surprised how unvertical are some walls even in new buildings, plus the room needs to be available at all times - not ideal to have people walking around between rooms / corridors to various scales / measures, plus you need a chair nearby if they have to take shoes off at the scale, then put them on again to walk away.