[Social Engineering] Why can't Observation be used to covertly judge approachability?

[NineDaysDead]

Originally Posted by Social Engineering, page 25

It’s possible to look a crowd over covertly, while seeming interested in something else. Concealing such an inspection takes a roll against Acting; Savoir-Faire or Streetwise can substitute for suitable types of crowds.

Why can't Observation be used to covertly judge approachability?

Originally Posted by Observation; basic Set page 211

This is the talent of observing dangerous or “interesting” situations without letting others know that you are watching.