If you already have tooling and knowledge for managing Postgres, and Postgres is sufficient for your queueing needs, then why would you incur the cost of introducing a new service? Most situations don't require the best tool for the job, and in that case, you are well served to use a tool you already have.
If you are using an abstraction layer over your queues (you almost certainly are) then you should be able to easily change to a different queue implementation later, should the need arise.