Stimulating the nipples can in fact lead to contractions. But stimulating the breasts to cause contractions is essentially an induction of labor, which is a medical procedure that is always done in a hospital.
It has been shown that the contractions caused by nipple stimulation are particularly powerful ones, more suddenly powerful than the slowly increasing intensities that the transition from false to real labor creates. The biology behind this is one of survival. After a baby is born, the uterus needs to contract down so that the gaping openings of veins and arteries can be cinched shut, decreasing blood loss. Nipple stimulation causes the release of oxytocin, which is the same hormone used for medical inductions. After delivery, oxytocin is released by breastfeeding. The effects on the uterus of breastfeeding immediately after delivery is a mammalian trick to make up for the method of live birth that would otherwise be a life-threatening blood-letting.
Stimulating the nipples to stimulate labor is backwards thinking, causing a contraction mechanism that was really meant for after delivery. True, we doctors often use this to do what is called a “Contraction Stress Test” to check a baby’s health in the uterus. And I suppose there are those who would direct their patients to continue an induction this way. (Several hours of nipple twisting and squeezing would not be popular method, though.) But the key here is that inductions are interventions into the natural course of events, and therefore need to be monitored. Doing it at home without any fetal assessment is extremely dangerous, because doing any induction of labor at home would be extremely dangerous.
Breast stimulation causing contractions before delivery is largely unknown by many people, and obstetricians generally would hate to see the breasts fall into the wrong hands.