This is akin to Why do my monster minions all have obvious weak points that instantly kill them?, but with one important difference: these monsters developed more or less naturally, they're animals that came into their present forms over years of Chaos Energy-induced mutation and natural selection.
Furthermore, small, relatively weak monsters (like Plops and Goblins) don't have weak points, it's only the big ones like, y'know, your typical dragon.
To make this even weirder, some powerful monsters don't have weak spots; things like boss-level ghosts and evil wizards. Most of these monsters are humanoids; in fact, chances are if a monster is related to or once was a member of humanity, they won't exhibit weak points. The few other monsters that don't exhibit weak points are things like Plopup, which lack a physiological weak point on their body but do have a single critical weakness (Plopup don't do fire, they don't regenerate when burned).
So, in essence, my question is: Why Do Naturally Developed, Powerful Monsters Have Obvious Weak Points? and only concerns those monsters that have these weak points.
In order for monsters to even have weak points, they should probably have some sort of advantage, or not affect one's chances of survival. The monsters who don't have weak points either lack a physiological weak point or don't, for whatever reason, ever develop obvious indicators of such weak points.
Basically, if magic is a natural energy cells can harness, why would it lead to magic-heavy creatures having glowing markings to indicate where their bodies are vulnerable? In other words, I'm asking why those monsters that have weak points would have them, when taking natural selection into account, as weak points would seem to impair one's chances of survival.
Going off this, please note that natural selection will likely affect weak points; they could end up in hard-to-reach points of the body, have some sort of protection that must be destroyed, removed, or worked around, or else be a relatively large vulnerable area instead of one crippling weak point. Also, monsters will be preyed on both by other monsters and adventurers, and this should be taken into consideration when answering the question.