[adrr]

Different breeds. Chickens we eat commercially are bred to to be that way. They can’t even walk without being in pain.

“ Most broilers find walking painful, as indicated by studies using analgesic and anti-inflammatory drugs. In one experiment, healthy birds took 11 seconds to negotiate an obstacle course, whereas lame birds took 34 seconds. After the birds had been treated with carprofen, there was no effect on the speed of the healthy birds, however, the lame birds now took only 18 seconds to negotiate the course, indicating that the pain of lameness is relieved by the drug.”

https://en.m.wikipedia.org/wiki/Broiler