I would like to put forth the possibility that the Uncanny Valley effect, is the Great Filter.

For a species to achieve the level of intelligence that we currently find ourselves imbued with. Social interaction is a necessity, to learn viable information from other members of the species.
If your species has "dumb" mimics, they would thwart the learning process, in whole, for social knowledge exchance.

A species must KILL their mimics.
This is both my answer for the "Great Filter" and "Why don't we see monkeys between humans and "dumb" monkeys?"

Because we get a flight or fight response, as a result of interacting with a "dumb mimic"

A burst of adrenaline, and the possibility to kill them.