In that context running postgres on K8S makes a lot of sense. You already have K8S and experience running it. Running postgres there makes it possible to share resources between databases and other applications. That improves utilization which means you can reduce costs significantly.
Another advantage is that unlike managed solutions such as RDS, you can use a more recent postgres version and postgres extensions that RDS doesn't support. Extensions such as PgQ or TimescaleDB or ...
Having said all of this. In a large organization, you have the benefit of economies of scale. Large fixed costs (such as developing the expertise required to run (postgres on) K8S reliably) can be amortized. It's possible for this to be a good idea, even a best practice for large organizations while at the same time a terrible idea for smaller ones. Most of the time, using a managed service like RDS is probably a better choice. In other words: You are not Google. You are not Facebook. You are probably not even Zalando. Figure out what's right for you.
JK, sort of.
My first go to is something like RDS, but I’ve run Postgres in k8s for pretty much one use case: everything else is already in k8s _and_ I need a PG extension/functionality not present in RDS.