Systemic homeostasis. Or, more specifically:
The goo hijacks existing bodily functions. If they are destroyed, it will “drop”, and attempt to begin repairing - just like it did with the initial revivification, where it grew into the space left by rapidly dying brain cells and took over. I can’t maintain full function until the damage is repaired. So a headshot can certainly down the zombie, since it will have to repair all that brain damage by shoving slime in the holes, but it probably won’t kill it permanently.
Any damage to a core component of the body (brain, spinal chord), or simply enough damage that the slime can’t afford to waste resources moving around and has to jump into full-on repair mode (heavy fire damage, torso shots) is enough to take them out of the action for a while.
Note that in-game right now, headshots actually ARE more effective at taking zombies out, they just don’t finish them off for good.