A pack of runners, breathing deeply in and out, the runners in behind are taking into their lungs some of what the runners ahead breathed out. There is a very low chance that any of the runners are carrying the infection, but not zero.
The risk of an individual event may be low, but multiply that by thousands and suddenly what you have is basically a CERTAINTY that someone will pass on an infection. If this continues happening, then the disease continues to spread. And it is still continuing to spread, if you follow the data.
In many hospitals, staff are battling this without proper protective equipment. They are risking their own lives and the lives of their families. Many food workers are doing the same, for low wages. Working without masks. Just do what you can. You should not run in a group, but if you are near other runners, you should be spaced WAY out. No reason not to.