The crowds are tight and packed.
With a few hours of intense marching, people naturally want to remove/adjust/lower the mask to talk/shout/run. Even with those wearing their masks, their effectiveness won't be 100%.
Which leads back to the OP's point - if the virus is airborne, why don't we see an uptick in deaths for that demographic?
Because the 'participating demographic' skews young.
“We estimate that susceptibility to infection in individuals under 20 years of age is approximately half that of adults aged over 20 years, and that clinical symptoms manifest in 21% (95% credible interval: 12–31%) of infections in 10- to 19-year-olds”
https://www.nature.com/articles/s41591-020-0962-9 https://www.cdc.gov/nchs/nvss/vsrr/covid_weekly/index.htm#Ag...
“We estimate that susceptibility to infection in individuals under 20 years of age is approximately half that of adults aged over 20 years, and that clinical symptoms manifest in 21% (95% credible interval: 12–31%) of infections in 10- to 19-year-olds”
https://www.nature.com/articles/s41591-020-0962-9
https://www.cdc.gov/nchs/nvss/vsrr/covid_weekly/index.htm#Ag...
That hasn’t been my experience. Every action I’ve attended has had clear messaging leading up to it about masks and social distancing, and attendees have adhered to guidelines.