Spear phishing, by design, is intended to not look like junk mail. Maybe it's designed to look like it came from an agent? Maybe like it was from someone they had hired to take care of their pets, or kids? If you're constantly busy (and it is my understanding that many celebrities are busy, at a bare minimum), and you don't want to think too hard about a particular thing, then a spear phishing attack could quite easily hit you, if your guard isn't up, and you don't consider technology to be a battelground.