Rabbitapril, we are so alike asking "Why?"
Yes they can't do reverse peristalsis at the top end. There's a muscular valve where the oesophagus meets the stomach, & it's so strong they can't even burp either!
Risking a guess I'm amazed by how good an instinct some of our pet buns have for the types of food they need. In the wild, the babies will also learn what to eat by watching the adults. (My bun learned an incredible amount of behaviour by watching me, & copying
) They have 26 different types of tastebuds (We have 4) & an incredibly acute sense of smell. I suspect that instinct & learning combine, so they don't eat poisons in the wild. So they don't need to be sick.
Again in the wild there's a huge variety of plants they can eat to settle down an "iffy tummy" before it causes big problems.
The lower gut is absolutely amazing & I think unique in domestic animals.
It does reverse peristalsis on a timed basis, to produce waste poops & caecs.