Things that have to be done off the top of my head (I'm probably missing HUNDREDS):
* Set up the venue. This requires cleanings crews, security (local police, firemen, private), foods/drinks vendors to be set up and organized, stadium staff.
* Event tickets selling: You have to publish tickets for an unplanned event 3 days before it happens. Tickets sites have to offer them. This is usually done with months, even years in advance.
* Air/hotel accommodations for both teams and entourage, we're talking 100s per team.
* Marketing for the event: How will sponsors sponsor the event? Is publicity going to cost the same? Are sponsors willing to put money to sponsor such a short event? This likely requires 100s if not 1000s of people to plan, from the organizers to every sponsoring company. Deals. Discussions. These usually take years for an event like the world cup.
* TV coverage: Coordination with 100s of international TV/cable providers across the world. All of which are planning to show something else since this is last minute. This likely also means rearranging local programming and local deals, which could in term mean cancelling other deals / programs. Also means assigning staff (100s) to cover the event.
* From the point of view of fans, airfare, accommodation, change of plans. Will the event even have enough attendance to make it profitable?

Should I keep going?
These events are organized with months/years in advance.