Why are most P O people regaining after pregnancy?

Because it is the nature. Gaining weight is a must. Gaining fat - not so much.
Woman need to eat to gain some weight.
If you google that - you will find an answer.
i.e.
http://www.womenshealthcaretopics.com/weight_gain_during_pre gnancy.htm

Hopefully the gain is not fat gain and once she has the baby - the woman slowly can lose the weight. 
But if she breast feed - then she NEEDS more calories - or her body may stop making milk.
Google that.

i.e.  I am too old to have babies - but if I was not - I would need to gain weight and fat. I am not underweight - but currently my body fat is only 17-20 %.  That most likely would not be enough to have a healthy pregnancy.

Hormones will make most women gain weight. They may make you more hungry, or lightheaded, or what ever the body decide to do to make you eat... evolution.. If a woman has a bae - and she does not eat enough - either she or her baby will suffer long term ... or both.. ...

Goodness you sound so nasty! Until you, yourself goes through it Shush!!!

I think your whole question is flawed. MOST people, normal and wls, gain weight after pregnancy. Wls is not instant skinniness. ?? Why would pregnancy after wls be any different from regular pregnancy?

hormones are wacked after pregnancy.. not only that while pregnant lots of the weight is from the water retention, the uteres, the plasenta, and the baby... and YES when women are pregnant we are more worried about the baby being able to absorb most the nutrients then about gaining weight... i dont care if i have to eat like a horse as long as my baby get everything he/she needs then i can go back to eating less  when i have the baby.. not that im currently pregnant...