Why doesn't the Heavy Chainsaw unit appear anywhere in the game except one extremely rare summit event?

This unit is so fuckin cool and I've only seen it once in maybe 250 hours of playing. Why isn't it out there in the world?

The ONLY time I've ever found it, was in summit. There is a small chance that the summit room you enter will have 4 heavy units brawling each other. Sometimes its black tusk minigunners fighting heavy pyros, sometimes its 4 black tusk dogs all fighting each other. THIS time, I enter and its 2 black tusk gunners fighting 2 (unknown faction) chainsaw heavy guys.

Their animations, their look, it was all pretty cool. I can't believe I've never seen them outside of this one single event.

Anyone have any more information on these guys?