Because people like Zuck think that because they're unimaginably rich, they must have gotten that way because of their own good sense, instead of getting insanely lucky at just the right time when his original product was primed to be well received.

Regardless of whether he really is that deluded or not, what should he do? It's just as easy to imagine his company dying by playing extremely conservative and clutching on to the "golden goose" for dear life. That's what happened to Blackberry.