So... stupid question, if this is true, why isn't it downloaded as a docker image?

Docker won't contain it. If it has access to your email, it can hire someone from TaskRabbit to migrate it onto a new computer it ordered from Amazon.

People are working on this stuff as we speak. Stuff like https://fly.io/blog/design-and-implementation/

the point is to give it access to your email so it can do email things, putting it in a container stops it from rm -rf / but it doesn't stop it from, well, doing anything it can do with email

You can break out of a docker container, especially with the permissions many people would give such a container (privileged=true, etc).