It seems dumb to you because you haven't figured out the reason for it.
It feels like the writers had this concept, a world where humans were no longer at the top, replaced by these unintelligent giants. Then they had to decide how to make this feasible.
In their deliberations they decided that it was unfeasible that an intelligent race with access to gunpowder would be so utterly powerless, a theme they very much wanted to explore, against an unintelligent race. So they had to figure out how to technically make them killable, but it can't be too easy because they really want it to seem hopeless. A bonus if the method involved extreme heroics. And so, cutting out a piece of neck from the back.
I'm sure they will explain it, as well as the construction of this ridiculously huge fuck-off wall. But it will be hard for me to not just see them as contrivances.
Still I will probably keep watching and report back if it changes my mind.