Open source created modern Oracle. I worked for a company built around Informix, and that database engine was one of the largest expenses of the company. I think we paid like $40k/socket in 2000. Today, you use an open source database to build your product.
So Oracle had to start selling integrated solutions. They do evil shit like pay higher commissions for salespeople to cross-sell their colleagues. So the Oracle Financials guy gets paid more to sell Identity crap than the identity guy. So the sales idiots form all sorts of weird alliances and kill each other.
Thing is, the vendor simply happened to build upon Oracle. If the software supported DB2, or more likely, supported a migration path from Oracle to DB2, I suspect we would have gone with that.
On another hand...
A lot of "Oracle required" software uses a ton of Oracle-specific stored procedures and other components, in fact, I have encountered an MRP software package that was written entirely inside Oracle database. All PL/SQL, with some bits of embedded Java.