You buy an Oracle DB because (a) you have a fleet of Oracle DBAs who agitate for you to keep Oracle, (b) plenty of software you want to buy only works with Oracle (or only has Oracle as the top-tier option) and once you've bought Oracle and hired Oracle DBAs it's hard to justify having other things, and (c) Oracle are very good at selling their product up the management chain; their enterprise sales teams speak the language people who run companies do, and IT departments don't.
(And I don't mean golf.)