They eat humans exclusively, even though they cannot properly digest them?

[He's fought plenty of man-eating monsters, but the rest...]

That makes no sense. Hunting a prey reduces their energy reserve, espcially one as large and likely to fight back or run as a human. If they cannot regain their strength from eating people, should they not starve over time? Or at the least be greatly weakened? Considering they possess stomachs, they obviously cannot be mechanical in nature, and they sound too much like animals to have a plant-like nutrition...