If you have thousands of lines of code relying on Oracle the cost to migrate would be enormous.
Imagine you're a new CIO. You know you're probably looking at a 3-5 year tenure at this new company and you want to lead with some big wins to set the tone and show your value.
You're reviewing proposals from your senior leadership. One of the options is an Oracle migration. It could cost a million dollars to migrate, but you'd save a million dollars a year going forward. Oracle runs your mission-critical internal systems, any issues with the migration and the system you migrate to is going to cause significant financial and reputation damage. You'll have to defend this decision if anything goes wrong, i.e. you've absorbed a lot of risk but a lot less upside to you personally.
What do you do? You put the proposal to the side and look for something that has a lot better upside.
https://aws.amazon.com/blogs/aws/migration-complete-amazons-...
I thought it was cool they retrained their Oracle DBAs into other roles as part of the project.