The historical record on the economic impact of mega events never, ever (and I mean ever) lives up up to the before-the-fact claims of consultants and the events' cheerleading squads. Why wouldn't, say, a Super Bowl have a massive effect on, say, San Francisco? Because San Francisco is already a vacation and convention destination. Having a Super Bowl in San Francisco will attract visitors, but it will also drive others to vacation elsewhere. The economic impact, however you want to measure it, is the net impact, not the gross impact.
Or, put another way:
But how can legions of football fans descend upon a region and not make a dent, economically? One reason is that measuring the number of people who head to a Super Bowl city for the game is a straightforward endeavor. Measuring the number of people who stay away from an overpriced, tourist-infested zoo is not. Especially in a year-round tourist destination like the Bay Area, Super Bowl visitors aren't flocking to hotels that would otherwise be empty; they're displacing would-be visitors. What's more, hotels in many Super Bowl cities triple their rates and insist on multiday packages. This drives away non-Super Bowl visitors and also leads to fans booking rooms for more days than they'll actually use — meaning those rooms aren't being occupied by actual people who could be spending actual money.
To claim an economic windfall based on visitor numbers without factoring in those who avoid the area or are pushed out "is like going to the hen-house, counting all the foxes, and saying 'Look at the economic impact of all these foxes here eating!'" Porter says. "You're not counting all the hens who are gone."