I think it's just flavor text. Or maybe they died of starvation and never forgot, poor souls.

I have Chickens hatched, and Boars, Wolves, and Lox, tamed, and breeding. Skeletons , since they are Friendly , when summoned, using Dead Raiser, say hungry, so, either it is a secret, on what you feed them, or, like OctoberSky says, it is basically a partially finished aspect of the end game content, as you also cannot upgrade many Mistlands / Black Forge weapons and Armor, YET, past level 2.

The galdr table is currently limited to level 2. Level 3 and 4 - which presumably allows the dead raiser to summon 3 and 4 skeleton warriors - will come in future releases.

I mean, they are undead, so I think they are just eternally hungry.

Ah see your doing it wrong. You don't feed the skeletons, you feed yourself. Eat two eitr foods and then summon 2 new ones.