AWS engineers are trained to use their internal services for each new system. They seem to like using DynamoDB. Dependencies like this should be made transparent.
A lot of AWS services under the hood depend on others, and especially us-east-1 is often used for things that require strong consistency like AWS console logins/etc (where you absolutely don't want a changed password or revoked session to remain valid in other regions because of eventual consistency).
even internally, Amazon's dependency graph became visually+logically incomprehensible a long time ago