it's because it's basically a quirk of one protein, which is only really found in the brain and nervous system, which is well isolated from the environment. And

it's because it's basically a quirk of one protein, which is only really found in the brain and nervous system, which is well isolated from the environment. And while it's resilient it's not like it grows in any environment which isn't the brain. So while it's a big problem if it gets in, it's really hard for it to happen in normal circumstances (even eating it, which is the main vector in animals, requires you eat quite a lot or get ludicrously unlucky).