Because when you give potent opioids you saturate the patient receptors and upon wake up the patient describing a painful sensation will be given morphine that has then less receptors available to act.
Imho this is what people call hyperalgesia (which i think is dumb) it's just a matter of receptor availability and function.
The other problem is that pain isn't accepted as a normal part of the act of undergoing surgery; why are we treating a 4 out of 10 "pain" which should reasonably be qualified as discomfort with opiods? Do we take morphine at home when we have 4/10 headache pain?
Expectations have shifted way to much towards a 0 pain & 100% recovery on day of surgery.
You know what cause chronic pain? A steel blade and a weak mind. Anesthesia doesn't cause chronic pain surgery does. Now if you're really bad at gas you're not going to make things better but all the mental masturbation going on about how we are going to make or break the patients future life experience with this drug or that one is a big pile of Sht.
/end rant