The same motivation that drives some folks to study e.g. philosophy, even though it's perfectly well known that this will never make you rich: Because they like it.
You don't just e.g. found a company for the sake of money - you also do it because it allows you to do things on your own terms, it gives you prestige, it gives you the satisfaction of being 'successful'. In todays day and age, this is usually equivalent with 'generating wealth', but I'm a firm believer that this does not necessarily have to be the case.