It is legacy decisions going back decades. Thirty years ago, you did not have a wealth of database alternatives. You picked Oracle and built the business around it. More and more business processes accumulate around the data store, all using some proprietary Oracle extensions. Eventually, the thought of disentangling the dependency is so daunting you are locked in forever until an existential risk materializes.

In 1998 I worked for a small nasdaq company that had a successful software as a service product that was growing quickly.

We used Clarion and MSSQL7 on windows because it was cheap. Since we started making real money, some figured we could finally afford Oracle and Sun (back when they were different).

I was a junior so my job was to evaluate the migration of one of our sql servers to oracle to test it out. I talks with the Oracle team who helps people plan purchases. They took my transaction level (~100M/year) and size (1-2GB/year) and came back with $1M for the system. This replaced a functioning $10k server. And we had maybe a dozen that would have to eventually move.

When I told them the current server was $10k, they revised their estimate to $100k. I recommended we not move.

I left the company a little while later and I think they ended up buying lots of Oracle.

Companies have money and don’t mind spending on useless stuff.

In my experience, it is from technical management in medium/big companies you'll listen some good things about Oracle as a database product (regardless of its actual merits), like stability, scalability, compliance checks, and other "enterprisy" features (like database encryption). Also, it is offered as a default database option for many enterprise applications from their vendors. While many people points to Postgresql as "the alternative", in many places outside USA its commercial support is not available, or too limited. Other commercial alternatives (like MSSQL) have the (more or less) the same bad reputation regarding licensing costs.

A few years ago I had the head of a devops team at a large company say that the project I was working on should switch from postgres to a "real" enterprise database like oracle. This happened while we were having zero issues with postgres, it was a perfect fit for our case, and it wasn't even relevant to the conversation. He just saw that's what we were using and reflexively thought that of course we should use Oracle.

It blew my mind at the time. Oracle is so widely hated among developers, entirely justifiably, that this guy's take really shocked me. I've literally never heard another glowing recommendation for that company before or since.

My old boss literally said they don't trust other databases. I tried to push for postgres. But they insisted only oracle is professional. Our software only worked with an oracle backend. I no longer work there.

Oracle Cloud has really good price and many locations. That's why I use it.

Then it's probably business requirements.

Single-sign on, in-person support, certificated software, offering training courses to onboard people, undeletable logs, help with upgrading major versions..

All from a single vendor so you can pick up the phone, yell "fix it" and go on with your day.

government contracts