That's because Amazon wants to do whatever they like themselves... you apparently can get stuff to work by running your own masters (w/ citus extension) in EC2 backed by workers (Postgres RDS) in RDS:

https://www.citusdata.com/blog/2015/07/15/scaling-postgres-r... (note that this is a old blog post -- pg_shard has been succeeded by citus, but the architecture diagram still applies)

And me saying "Apparently" because I have no experience dealing with large databases on AWS.

Personally had no issues with Citus too, both on bare metal/VMs and as SaaS on Azure...