Because humans, generally, look the exact same. Two eyes, a nose, arms, legs, a body.
What you're asking for is a system that has a copy of EVERY animal (from squirrel to giraffe) and a way to compare EVERY object on your screen to EVERY animal in all of the animal kingdom within a split-second to warm you that it's an animal.
If they had this system, half of the time it would tell you that a tree branch was an animal.
2012 328i Sport Line: H/K + SA