If they had "no problem" cannibalizing their offspring, they'd quickly go extinct! In reality, cannibalizing of offspring only occurs in particular circumstance

If they had "no problem" cannibalizing their offspring, they'd quickly go extinct! In reality, cannibalizing of offspring only occurs in particular circumstances - if conditions are poor enough that most of the young won't survive anyway, for example. In which case their instincts kick in to cannibalize as a way to recoup the energy and try again at a better time. If conditions are right there will instead be a strong instinct to protect the egg sack / young.