Because they don't exist.
Yes, they do. They saw real Yeti in the end. Far away. And they were more docile than the robotic ones.
If I were the Doctor, I'd suggest recruiting real ones to help agains the robotic ones.
They don't exist. Yeti/Abominable Snowmen are myths.
In Doctor Who, they are real.
In the real world, there is evidence there is. Even David Attenborough confirms it.