Couldn't the same be said for any major area that attracts millions of visitors each year? Before covid ~30 million people were visiting Paris each year from all over the world.
College Spring Break, with congregations in Florida, Las Vegas, and Mexican resorts are also as concern. Florida spring-breakers featured early in the US Covid outbreak. US spring-break revelers vacationing in Mexico were an early phase of the 2009 swine flu outbreak.
https://en.wikipedia.org/wiki/2009_swine_flu_pandemic_in_Mex...
I've been a visitor during a World Cup and its not just the football people go for.
There's even a term for it: World Cup fever, not just isolated to world cups, many other sporting events have the same problem of being super spreading events for STIs https://sexualhealthbucks.nhs.uk/latest/world-cup-fever/
I'd also say prolonged contact and mingling is worse then a short one like above, plenty of time to pub and club crawl, sleep with multiple partners, allowing disease to expell after its normal 5-6day incubation period.