We self-host our Oracle instance because we really, genuinely like Oracle SQL and Oracle Cloud is the only cloud provider (that I'm aware of) that offers Oracle as a service, which is undiserable. Also, in general I personally like the fine control of being able to know the condition of our whole database, including state, deep profiling, and our backup situation, as someone with an old-school sysadmin mindset. However, it's relatively straightforward and well documented[0][1] to migrate to solutions like RDS from existing self-hosted installs if that's what's required.
[0]https://docs.aws.amazon.com/prescriptive-guidance/latest/pat...
[1]https://aws.amazon.com/getting-started/hands-on/move-to-mana...