While people are very good at planning long-range strategies, they can't do it while simultaneously fighting off a horde of angry monsters. Human perception is very limited. We can't see behind ourselves. We can't see around corners. We're much better at detecting moving objects than static objects.
Perceptual attention is also severely limited.
When fighting one enemy, we are much less likely to even be aware of another enemy approaching. Finally, human short-term memory is extremely limited. When people are distracted from one task, they often forget their previous tasks completely. Real military doctrine is based in large part on exactly these kinds of limitations. On a tactical level, soldiers do everything possible to avoid giving away their positions.