Never thought of that. Seems odd we seem to have an "outbreak" and yet they haven't? It's just wierd to me.
It actually makes perfect sense. The virus doesn't really transmit in outdoor settings or in settings with strong ventilation because it requires sustained exposure to the particles (this, for the record, is also why wearing masks on airplanes is moronic, because the plane's filtration system is so strong that there's an almost 0% chance of transmission, even on a long haul flight in a packed A380 where the person in the seat next to you is positive). When you think about a football stadium, from a players perspective, they're basically open-air environments both because of the amount of open space around them in every axes except down (true of anywhere that isn't employing jetpacks) as well as an immensely strong HEPA filtration system in the buildings themselves. Opposing players, over the course of a game (with the exception of linemen) are seldom in the same space as others for more than a second or two at a time. Obviously, the dynamic for the fans is significantly different, as they spend 3 hours in close contact with at least 25 other people.
Also, as a byproduct of their job, these players are all wearing mouthpieces, which means they're all breathing exclusively through their noses, which offers additional filtration.
So the issue with cross-team contamination isn't being on the field at all. The issues come to be as a result of many of these people from opposing teams actually being friends with one another, and meeting up after the game for a drink or a meal or even to the home player's home.
Within an individual team, the greatest risk comes in the form of individual groups sharing a room for things such as film study or whiteboard strategy. This is why you'll see the entire secondary or the Browns QB room get wiped out, but the linebackers will be generally fine.