Some of it might be that, but some of it might be more prosaically the same thing that keeps people playing competitive games on one hand, or cookie clicker on the other. (Cookie clicker could read as "pure greed" if you squint.)
They are all psychopaths, devoid of normal human emotions.
Some of the behaviour stems from their situation itself.
The huge difference here is that for these billionaires, everybody else is a "far away consequence". It's as if I kept tossing my garbage into my direct neighbor's window and flushing my toilet into their kitchen sink.