[AppyPappy]

Because they can't talk.

[stanz]

Archaeologists tell us that things get invented or discovered when there is a need for it.Primitive tribes whose ecosystems remained the same for hundred of years had no selective pressures on them and so, they had no need of making changes. Their tribal customs, no need for written language and other cultural practices remained exactly as they were for ages.
It is the same for animals. Chimps in the wild do not need hospitals. Humans need them.