The reason is because most of this stuff isn't necessarily important. An overly detailed description can very easily become like slamming on the breaks when it comes to pacing. Everybody has their own happy medium, whether it be vague descriptors or cherry picking some details they think help the reader picture the character.

Personally, I think it looks silly when you start bringing numbers into descriptions. Like "Mary Sue was 6 foot 7 inches." or "John Doe was 152 pounds." This isn't crazy unprofessional or anything, i've seen published writers I really like do this plenty of times, but for me personally it always seems clinical as fuck. The weirdest I seen is B/W/H measurements for women. 

My own way of doing descriptions is to pick out a few features that stand out and otherwise stay vague. Something like "Robertito was a stocky, big-boned man with an oily handlebar mustache and eyes that gave the impression that he was way too clever for his own good."