Because operating a database sucks. Specifically: setting up database replication sucks, setting up failover sucks, setting up backups sucks (even with a PaaS

Because operating a database sucks. Specifically: setting up database replication sucks, setting up failover sucks, setting up backups sucks (even with a PaaS you need to do this, but you can use it as your first layer), migrating database clusters to new versions or hosts sucks, keeping your host's software up to date sucks, and setting up alerting and monitoring sucks. It's easy to get an open port to connect to, but hard to keep it there for five years.