Limited-time promotions, e.g. a 10% off sale, work on short timescales that are easily measured. You can easily say what the ROI is of an advertised promotion, and it is often positive. (Side note, these promotions have a more-difficult-to-measure detrimental effect on your long-term profitability, closely related to the point I am about to make below)
Brand advertising, in contrast, works on a spread-out scale of years or decades. When Coca-Cola runs a polar bear ad at Christmastime they don't do it to increase sales of Coke that week, nor should it be measured that way.
The actual effect that a brand advertisement has, is to add PENNIES (not dollars) to their sales every day, for the next 100 YEARS. It's a long-term investment. And for these reasons hard if not impossible to measure or control for confounders - but that doesn't mean it doesn't work!!
And without that brand investment, their brand value is continuously eroding away, at a rate of pennies (or more) per day.
Source: Been on the leading edge of marketing and advertising for over 12 years