1) costs, 2) ultimate observability and performance tuning - with self-managed Postgres, you have full control and can tune it better, monitor all the details, put pgBouncer or Odyssey on database nodes, there is no need to spend hours and sometimes days or even weeks with support engineers trying to understand what's happening under the hood and how to fix/improve it (but you need skills -- if you don't, managed service support may be beneficial, of course if you're lucky to reach experienced engineer), 3) specific need: have replication connection from outside, control backups, etc.

That being said, the benefits of managed offerings are clear and sound: you don't waste your time for many DBA tasks, paying some extra and losing some control. Many companies choose this. Some change their mind over time and move back to self-managed, or use hybrid approach.

One more thing. If you need to maintain dozens or more databases, you need automation. This leads to either managed offerings, or k8s (look at StackGres.io).

There is also https://github.com/CrunchyData/postgres-operator which seems more mature.

All of them look interesting and developing rapidly.