I think the root reason for this is AWS stacks have to authenticate from a single origin (i.e. user credentials) instead of Terraform which can utilize multiple

I think the root reason for this is AWS stacks have to authenticate from a single origin (i.e. user credentials) instead of Terraform which can utilize multiple auths. This makes it necessarily complicated for AWS stacks when it tries to deploy another stack in another account, as the stacks are also account based. (but I imagine terraform stacks isn't).