Why monsters can develop this skill, while PCs can't?
You took the words right out of my mouth. Personally I don't like it. For the vast majority of the monsters it is really used to replicate superior armor/toughness not some innate ability to avoid danger. For some reason the developers didn't want to give any armor more than a +60 bonus. (For example, Dragons, have the +60 armor, then whole buttie-loads of "Survival Instinct" to give them a ridiculously high DB. I say, just allow the larger, thick-skinned/armored monsters have larger armor bonuses than can be had by "people" wearing armor.)
To this I say....humbug! I can reason in my head the ability of a huge, scaled monster to develop a natural toughness that makes it harder to hurt than a "person" in plate armor. I cannot do the same with a trained, mental ability to instinctively understand how to best protect themselves in dangerous situations. Any and every being that routinely encounters hazardous would learn the same ability, or die.
That AND put in the Combat category the Survival Instinct skill (or, make it outside any skill category so that it always costs 4 DPs). Personally, I have been looking at using the Chi Defense skill as a template only instead of the flat +10, their normally determined skill bonus is added directly to their DB, with the -20 to all other actions while using Chi Defense. Or, use the modifier as the bonus on a skill check on the bonus column to determine the actual DB bonus granted.
Or you could just say that every so many levels (whatever you, as GM, decides, I suggest every 5th level, i.e. 5th, 10th, 15th, etc.) you can repurchase the
Instinctive Defense talent.
I know that ICE has the tendency to instinctively go with the grittiest, least PC beneficial, method but I suggest we all remember the name of the game
HIGH Adventure Role-Playing.