Because if what you really want is just an NPC with the specification "must have high skill modifier", then do that. Screw all that enhancement bonus, synergy bonus, feat bonus and whatnot, and just give the guy +12. Nobody is ever going to notice. You said it yourself:
Why do NPCs need to be fully statted? So, the healer with Heal +12. Would it matter if his stats in a 3.5 adventure read:
...
He still follows the basic rules, but I only wrote up what I need for him.
And all you need is that +12. So make up your mind. Are you following the rules, or are you ignoring them?
Precisely. If all you are doing is slapping on special abilities willy-nilly, then why bother with complicated formulas like "BAB = HD x 3/4", "1 feat per 3 HD", "8 skill points/level", "+1 hp per 2 Con", "class skills are X, Y, Z, W" and so on?
Everything is possible if you ignore the rules that you paid money for.