Why birds and mammals don’t show cannibalism, but most other animal clades do? Even intelligent reptiles like comodo dragons can cannibalize their young. Some small mammals may cannibalize their altricial, immature young or deformed young for economical reasons, but the hunting of fully alert and moving conspecifics is rare. Even more solitary species like cats don’t eat each other. Why?

Interesting question.

I am not actually sure how true your observation is though. At least anecdotally (not sure if there is hard data) cannibalism is commonly reported in wolves, although the primary motivation for killing a conspecific may be territory defense (rather than food). Cannibalism has also been reported in chimpanzees, and many birds commite siblicide (although don't necessarily eat dead siblings).

agreed and very many rodents will eat their young if stressed.