In many cases the decision to go with Oracle was made decades ago, and the cost of migrating large, legacy databases and codebases away from Oracle is higher than the cost of continuing to use Oracle. Oracle licenses can be very expensive, but migrating and rewriting and retesting working code can be even more expensive!

Also, Oracle is pretty good at achieving vendor lock-in. For example they introduced a rapid web application development platform called "Apex" [1] about a decade ago and shipped it with every Oracle database (even the "free" Oracle Express Edition). In my experience, developers who use it tend to like it, it does achieve rapid development, and there's really nothing else like it outside of Oracle. Once an organization has Apex apps, though, there's no migration path to Postgres that doesn't involve completely redeveloping the Apex applications from scratch.

Finally, despite all of the ways in which Oracle seems to be business and developer unfriendly, for a developer or DBA trained and experienced and entrenched in the Oracle ecosystem (and for whom license costs and sales tactics are not a concern), it's really not such a bad place to be IMHO. I personally would not choose Oracle for my own ventures but in the cases where Oracle has been chosen for me, I enjoy working in the ecosystem.

[1] https://en.wikipedia.org/wiki/Oracle_Application_Express