There is, though - you need its value to float on the open market so that if ads are underpriced (relative to their annoyance to the user), more users opt out, the supply of BAT on the market declines, advertisers have to pay a higher price, more users opt in, and equilibrium is restored. That's the whole point of having BAT in the first place: correctly price attention via market means.

Similarly, you can't use a regular currency (like just paying users in dollars, AllAdvantage style) because the value of a dollar is more dependent upon other markets like housing or food than on advertising. If attention is mispriced with that system, the only party who can correct that is Brave, and Brave doesn't has neither the information nor the incentive to set a correct market-clearing price.