For the most part no one has claimed a non-medical mask can reduce one's individual risk of catching the virust, it just reduces the possibility you'll transmit it. Even a poorly fit mask or a flimsy bandana (ideally still combined with distance). It's a matter of degrees, and, particularly, a matter of people collectively wearing masks. If a supermarket cashier waits on 50 people during a shift and 10 of them aren't wearing masks, they've endangered both the cashier and the other 40 people.
I'm not sure the argument for herd immunity is any better than the hope for a vaccine, i.e. you can catch the common cold multiple times (and even more so from year to year). And the flu vaccine is often less than 50% effective. So far they believe COVID-19 doesn't mutate like the flu though.
If there's no vaccine for two, three, four years - or ever - life will have to go on. But that doesn't mean there has to be sports this year. Particularly baseball and football where they may never finish the season, if they can even really start it.
People have already died who never should have died and one death in a PR-conscious, liability-averse entertainment business would be one too many. That the numbers say it's not fatal, maybe even not that damaging, to people who aren't older or suffering from other conditions still doesn't make it okay (if not morally than as a business decision).
It will be interesting to see what happens in the next few days - is this the opening for the owners who don't want to play a season, or is it the opening for the owners to think they can win a future grievance. If the Phillies have 20 positive tests out of 32 but nobody gets truly sick, is that the blueprint? Is MLB itself a test for herd immunity?
I don't think anyone is equating keeping the population fed with playing a sport but most so-called essential workers had no choice about whether to work or not, and the players do.