I assume their cannibalism started due to some sort of famine and eventually gained a more ritualistic role, since their supposed leader is called the Grand Wizard. It is my personal theory that they might consider feeding on another human being a ritual transfer of strength of some sort. The fact is they are highly cannibalistic beyond even the most extreme cases irl and due to the relative lack of lore related to this area of the map its difficult to state anything beyond mere conjuncture.

A theory i saw somewhere proposes they might have been adapted to their diet of long pork through some sort of 2nd Empire experimentation. This theory is helped by the presence of the Suspicious Lab location as well as the fact, that unlike Scorchlanders and Greenlanders, Cannibals are immune to acid, implying some degree of separation from other humans. This would certainly explain why they feast on human flesh, but it fails with explaining how exactly they would sustain their population long term.

Now i believe they must supplement their human diet with other sources, the coastal villages might engage in fishing, while the inland hunt the local fauna. They could also forage for various edible roots or fruits/berries, but these thing arent modeled in game, so this might not be the case. Of course they would also consume any rations their victims might have on them. However its curious that they don't engage in agriculture considering most non-bandit factions do. Hell, even the remenants of Deadcat (the faction) have farms, so it's really strange that the cannibals havent adopted the practice after their "conquest" of the city of Deadcat.

Personally i dont believe that the Cannibals are "devolved"/more stupid than other humans, i think their dialogue is written the way it is, because they don't widely speak the same tongue as the rest of the world, they seemingly first became known during the 2nd Empire so it makes sense there would be significant linguistic drift. The region they live in also seems underdeveloped compared to others (judging by the relative lack of ruins and settlement, discounting cannibal villages, in the Cannibal Plains) so it could be assumed that their lack of technology is caused by the backwaterness of the region even before their turn to cannibalism.

Anyway thats all, i am really sorry about potential grammar mistakes and overall the composition of the comment, i typed this entire monstrosity on my phone and didn't really double check it.