Unfortunately, in the USA, there are significant financial and legal barriers that work against such a seemingly logical solution. Thus we have the airlines in control, since they spend the money. Just about the same amount as if it came from taxes or airport fees. Some might argue that having private enterprise making the decisions to upgrade facilities and them managing the upgrades is a bonus. Unfortunately, with municipal airports we get the worst of two worlds.
Airlines end up controlling gates, which is both inefficient and a barrier to competitors.
The same improvident bureaucrats who would have been spending the tax dollars end up in control of terminal construction. Someday, US airports will wake up. Don't get too excited about the prospects too soon. Before You Go . Cheap Charlie appears every Monday on this site.