Okay, thanks for sharing your thought process on it. In other words, spikes are the run-of-the-mill 'Stone Age' traps that every player character can make, and unlike more advanced traps, nobody masters spikes.
But following that logic, why should you get XP for kills with a wooden club or wooden bow? These, too, are unspecialized Stone Age weapons. The game instructs every player to make them, and players don't master these, they master the more advanced weapons. Among primitive survivalists, hunting with weapons and hunting with traps are both valid strategies (not quite the same thing, but there are no zombie survivalists to consult

).
To be clear, I'm not trying to prove a point that spikes
must earn XP. Rather, I’m trying to tease out if the underlying vision for what makes something worthy of earning XP is self-consistent.