Because everyone involved in the design, writing, and concept of the Cybusmen was a complete idiot.Well, you see, rather than being similar to humans, the original Mondasians were like the Silurians and had a third eye. Much like how they kept a face as they changed and developed over time, they also kept this third eye as a cultural thing. Then they did the sensible thing and stuck a gun in it.
Then why is there anything up there on the Cybusmen?
Ermmm... no. Going back to The Tenth Planet, it clear that the 'thing' on top of their heads was a light - akin to a miner's helmet-lamp. This is also evident in The Moonbase and The Invasion...
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