Now I'm not saying a sharpened wooden staff wouldn't be able to kill you because in truth it could but we see them poking the heads and the weapons just going straight through the skull to the other side. the skull is as hard as concrete and unlike flesh it doesn't just rot away. Bones get weaker over time I mean if you grab a bone in a museum that was buried centuries ago you could likely damage it by touching it too hard.

Skeletons from people who lived in the stone age have been found with missing pieces because some of the bones just broke up over the years, but bones don't just break down in a few years it takes centuries. so in reality when a walker is stabbed like that in the head it will probably just have a hole in the flesh and maybe a crack but the brain should be safe. To really destroy it you would have to do what negen did and just hit them with all your strength on the head and probably the few times too.

The gas is the same it doesn't apply to the real world and this isn't the writers and directors fault because that is just how it works. other shows and movies do this, in fact, I would say all of them do it. even star wars a show based around science has tons of incorrect information about physics, for example, the death star when it exploded we heard a loud bang and the spaceships constantly make noises in space. in real life, all the space battles would be silent because sound cannot travel in space.

There is a term for this something along the lines of disbelief. The audience doesn't really care about how accurate the story is to real life physics. if its a good story and it works no one will care. Now there are limits to it like rick cant just sprout wings and fly off there has to be some limit to it.