Actually there are (Ebola, haemorrhagmific fever etc) are mostly deadly and easily transmissible - however they tend to be self limiting in part because they are so deadly and (luckily) because they aren’t airborne and thus distance … literally the distance between groups of people helps limit them. (Also the fairly short incubation period).

However all of that is irrelevant since the GP was musing on the “huge genetic diversity of humans” being a barrier to xenomutation of animal viruses to humans.

Thats not the case, because humans are pretty much genetically identical to near a 1000th of a percent - so a particularly virulent virus that mutated from an animal to a human because of an organ transplant has a pretty good chance of causing a major pandemic.

You don’t even have to take my word for it, Covid, swine flu, h1n1 are all examples of animal infections jumping - and that’s without the added long term exposure / selection pressure a transplanted organ would provide.

Caveat: I’m all for this kind of research and development given the lack of transplantable organs in many health departments, but I’m also in favor of r&d for artificial replacements too.