What incentive does a rich billionaire have for starting a business? Money? They have billions.

The only business they start are for themselves / the benefit of other businesses they own. Their wealth accumulates, and it is spent only to perpetuate its growth. It does not find its way into new businesses. It does not create new sector growth.

However, the people that want billions and don’t have it have a hell of an incentive to start a business and run it well.