Lots of parents lie to their children, repeatedly and ardently, about the existence of Santa Claus. Why? Santaism imposes large costs on parents, both the moral costs of lying and the opportunity costs of losing credit for well-regarded gifts. Yet Santaism does not appear to offer countervailing benefits. You don't need that mythical construct to tie good behavior to good gifts; parents routinely use carrots and sticks to get their kids to act appropriately. As d said of Santa Claus last night, during our long drive home from a dinner party, "What's in it for the parents?"
I can think of a few answers, though I don't regard any as very satisfactory. Herewith a quick survey: 1) Unthinking adherence to custom leads parents to ignore the unimpressive cost/benefit ratio of Santaism; 2) Convincing kids about the existence of Santa offers parents the benefit of exercising over their children the power afforded by superior knowledge, a form of power useful in other areas of socialization; 3) Parents who feel anxious about their ability to provide pleasing gifts regard Santa as a form of insurance—he offers a convenient fall guy.
As should be evident, we don't practice Santaism in our household. It's not just that the costs outweigh the benefits for us parents, either. We think that exploiting the trust of your kids, setting them up for an inevitable and heart-breaking disappointment, imposes large net costs on them.