In Hollywood they have a saying: "Nobody knows anything." This explains how huge-budget, big-star, focus-group-approved movies manage to bomb on a regular basis.
It is not dissimilar in advertising. Most of consumer behavior is perfectly obvious -- people like things that look nicer, taste better, work more dependably and cost less (see Salesmen & Sociologists.) But then there’s the mysterious emotional part. And what we claim to understand about the mysterious part is usually speculation and ideology masquerading as knowledge.
Our clients don’t want to be told that they’re paying for speculation and probabilities. They want data and they want it now. No wonder ad agencies spend great quantities of time and money dressing up probabilities to look like facts.