I agree with you - and you are still missing the point. The enterprises are buying a service. They don't care that it is running on AHWRGGG instead of TLMWBBB (which is soooo much better). What they care about is that it runs, and when it doesn't, that someone fixes it. This is it. Could you run everything on PostgreSQL instead of Oracle? Yes, take a look at EnterpriseDB. I guess Oracle just sells better? (guessing here, no idea)
The money is of course important, but these providers are smart. They take only what they can and not more. Which is still big money. And while Oracle & co. would never make it into any company I can make a decision for, I don't think that keeps them awake at night - there are enough (big!) fish in the ocean.