It is common for parents to imagine their children as carefree, for their troubles will ultimately pass and appear meaningless to adult lives. Yet if their children were truly so happy, grieving parents should note, then Sandygast would starve. After all, it is rare for adults to spend their time on the beach building sandcastles, or to be too weak (in body if not in spirit) to escape.
Sandygast seek out the miserable for the same reason living predators seek out the old, young, sick, or weak; escaping a ghostly sandcastle requires a great deal of willpower, and not everyone has an adequate supply. Admittedly, ghost pokemon do not "eat", but they must still seize something to maintain their presence in the world of the living. Perhaps Sandygast, who rarely led happy lives, are well-attuned to negative emotions and see their actions as mercy killing, or perhaps it's some kind of spectral niche; surely, when others safely use a particular shovel, it does much to overcome their victim's caution.
It seems remarkable, given the existence of Sandygast, that Alolans, like people elsewhere, go to the beach at all. Let alone that they dig sandcastles, which many an outsider compares to playing with fire - but anyone with a beloved Charmander or Ponyta has themselves, in a sense, "played with fire." Alola may be a large region, but one is rarely too far from the beach; even when parents forbid their children from coming with them, Sandygast can possess the shovels of construction sites as easily as those of sandcastles, and still rarely lack for nearby sand to build a body to draw in curious children.
One can not hide from Sandygast; one can only avoid being swallowed by the sand with a spirit strong enough to drive them away.