We do have to start opening up at some point. But masks and distancing are still a good idea as things open up. From what I saw on TV news, both were largely missing at one of those big sporting events.
As to the claim that "enough people have already been vaccinated": According to the CDC
https://covid.cdc.gov/covid-data-tracker/#vaccinations only 19.4% of the total population and 24.9% of adults over 18 have been fully vaccinated.
Until every "innocent" person who wants a vaccine has gotten one, people going out in public without masks and distancing are still endangering others.
And yes, unvaccinated people can avoid going to huge sports events or the like. But this does not protect them from the other people who go to those events, catch the virus and bring it back to their home neighborhoods, workplaces, etc.