Prions don't appear from thin air - and you can already get deadly prions eating meat, the reason why you don't is the same why you don't get all kinds of deadl

Prions don't appear from thin air - and you can already get deadly prions eating meat, the reason why you don't is the same why you don't get all kinds of deadly diseases from meat - it's (usually) well regulated and any outbreaks are immediately culled. A pig genetically modified to be "more like a human" wouldn't spontaniously get deadly prions.