Why are posed newborn sessions done so early?

Posed newborn sessions are done when baby is 7-14 days old. Newborns younger than 14 days old sleep through almost anything and are naturally curled up so sleepy posing is usually possible.
At 14/15 days old, baby goes through a developmental growth spurt that makes them very fussy, easily startled, uncurled, and acne/flaking sets in.  Growth spurts mean non-stop cluster feeding and stress for the mother, so it is important to come in early, before that difficult age begins.