Why are people cannibals?

As a rule - no.

Even in those areas where cannibalism was regularly but rarely practised, it was always more about the ritual than the actual meat. A recurring thought was that if you ate your defeated enemy, his strength became yours.

The 2nd most common reason for cannibalism was sheer necessity. People getting trapped somewhere and slowly starving.

There are several well documented cases where either survivors began to feed on those who died first, and even when someone was chosen and killed to provide food for the remaining.

Then of course, there are always the crazies.

Someone w/o any better reason than "because I want to"´who have eaten parts of another human.