The fundamental problem here is that very few sales are 'cold and direct'. Very seldom does the funnel work like: 'Target sees and ad, buys the product or not'

The fundamental problem here is that very few sales are 'cold and direct'. Very seldom does the funnel work like: 'Target sees and ad, buys the product or not'. All ads ultimately have a lingering buy effect, and all ads are essentially also a form of brand advertising. When they see your 'test ad' the first time, it makes them more or less likely to buy your product at some future date, the next time they are in-store or whatever. For some kinds of things what you describe works, but for other kinds of things, and for bigger companies, not so much, or rather, it's harder to do.