Cannibalism is considered to be taboo because it means that people are eating people in order to have food. Sometimes, it can be understood when a group of people resort to cannibalism because they are starving and the people around them have died. They have no other means of nourishment for their bodies. However, if there are other ways to find food, then resorting to cannibalism is considered to be wrong.
There are tribes that currently resort to cannibalism. One tribe is called the Korowai tribe that lives in Indonesia. They eat other people due to their tradition. Due to the thought that people around the world frown upon this tradition, fewer and fewer people from this tribe have been continuing this tradition.