Why no tires made of Vibram?
Vibram is simply a brand name for rubber soles made by Vibram S.p.A. of Italy. The actual composition of the rubber and the lug design used differs across their product line, with specialized compounds for hiking, industrial use, and for their "Five Fingers" barefoot running shoes.

They began as a manufacturer for hiking shoes and were one of the first to make them with rubber soles, which helped them become a well-known brand in that market.

The manufacturers of MTB tires are continually experimenting with new rubber compounds and balancing durability, weight, and traction. I'd expect that Vibram S.p.A. doesn't see much opportunity in becoming a bicycle tire manufacturer.