Clonally transmissible cancers are typically only seen in highly inbred populations. Tasmanian devils qualify because they went through a population bottleneck when they first colonized the island. Dogs qualify because of human breeding. (It's pretty unlikely that CTVT could be transferred to a wolf, even though pet dogs and wolves are the same species.)

The reason most cancers aren't transferrable, is the same reason that organ transplants are so hard. Most individuals in most species have unique antigen profiles. The immune system attacks any foreign bodies without matching antigens. An internally generated tumor works because it's cloned from an individual's own cells.

But a foreign tumor, is very unlikely to have a matching antigen profile. Therefore the immune system will attack and destroy with extreme prejudice. However a highly inbred population may lack the sufficient genetic diversity to have individually unique antigen profiles. In the extreme case if we were all clones we could pass tumors back and forth with ease.

The short answer is that it would be virtually impossible to see any widespread cases of clonally transmissible cancers in non-immunocompromised humans.