I would bet they did in fact sell 5 million tickets, but that many of them were bought in anticipation of being resold on Stubhub type exchanges for a profit.
Hotels and flights are a better gauge of how many people will come. Maybe many of the cities hosting games will be viewed live by people who can drive in and out for the day.
In any case, the free market drives it. Air and lodging is expensive and a bit of a hassle especially for foreigners trying to get in. I don’t know exactly where each of the major teams are playing or where the brackets will be located but it may involve 2 or 3 flights for each person who wants to come in. For example if you had to fly from Dusseldorf to London to New York to Wherever in America. That’s closing in on 24 hours just for travel.
I couldn’t care less, but the whole affair seems like BS to me…..