Just because something is available for free doesn't mean it's better for all use cases. There are cases where Oracle will perform better than Postgres (and vice versa of course).

And there's a business case for finger pointing

security issue with your open source DB install? It's either your fault (configuration), or the fault of some possibly volunteer engineer (bug). But if you pay enough, the whole thing is Oracle's problem, and you can tell investors with a straight face that it's not your fault. And Oracle are big enough that it's an easy decision to defend should something go wrong (which is something of a self fulfilling prophecy, but that's the way it is).

But yeah, whenever I need a database it's Postgres :)