The first child is the "practice child". Parents "learn on them".
Most new parents apply everything they've heard and read about raising children on that first child. In some situations the oldest child is held responsible for looking out for their siblings lit a lieutenant in the military obeying orders from their captains (the parents).
When parents want a "date night" they leave the oldest "in charge".
Anytime something goes wrong they're told: "You should know better!" As if wisdom is imparted according to birth order and not life experience.
By the same token being the "first' you're likely to have more childhood photos taken of you than your siblings, there are no "hand me downs", you're encouraged to demonstrate advancement for your age by talking, walking, and reading sooner than other children. Everyone wants to shower you with love and affection (until) your parents have another child.
Some older children adapt well to becoming the older protective brother or sister and some ponder ideas for eliminating the competition. :)
The middle child has the experience of once being "the baby" or youngest until they got "demoted" when another child joined the family. So now they're not given the respect and responsibility or expected to achieve as much as the oldest child and they're not being pampered like the youngest child. Oftentimes the youngest child will look up to the oldest child and not have respect for the older middle sibling especially if they're close in age.
The biggest difference for why children are treated differently is because parents like anyone else change and evolve over the years.
A 34 year old mother is not going to "sweat the small things" as much as she did when she was a 26 year old mother. In fact she may have reached the realization that children grow up too quickly!
She now wants to hold onto all those moments when children love being with their parents instead of being embarrassed to be seen with them.:) Therefore she's in no hurry to cut the apron strings.
The thought of having an "empty nest" also scares a lot of parents!