Because they might want to run some of the applications that Oracle sells that run on top of their database - the example that I am most familiar with being Oracle Hyperion HFM which is by far the leading financial consolidation application for large corporations. Even Google uses Hyperion:

http://www.google.com/intl/en/jobs/uslocations/mountain-view...

[If 12 dimensional hierarchical databases with complex logic are your thing then Hyperion is really rather cool in a perverse kind of way].

BTW If anyone here is interested in this kind of space I have had some interesting experiences in building extensions to HFM...