Part of the difference is going to be diagnostics. There are very clear diagnostic criteria for measles or whooping cough that allow them to be identified with an office visit, so it is possible to provide timely notification to parents about those cases. The same doesn't apply to COVID19. You can't go see your doctor and find our immediately whether or not it is COVID, there are enough shared symptoms with other viruses that it would be irresponsible to report to families based on symptoms alone, and test results to confirm the illness are lagging by several days to a week at this point. And that, of course, assumes the person who is sick is tested.
COVID protocols are more likely to resemble those relating to the flu. Our school does send a letter if they're notified of a confirmed case of influenza.... but since most people don't get their kids tested for flu, those letters don't even come close to notifying families about most or all cases. In fact, I think I've only seen two of them in 9 years at the school, and I know we've had more kids get the flu than that!
Our state athletic association "postponed" (why can't we just say cancelled? they're not really going to run football at the same time as baseball and track in the spring!) football specifically because of this problem. It is very, very hard to accommodate either masks or distancing in football. Other fall sports - cross country, swim - are going forward, but football specifically is such that even the minor breathing hindrance of masking and the minor interference with the range of vision is a problem, and obviously there's no distancing in a contact sport. The kids have to have a certain number of full-contact practices, to focus on safe hitting techniques and other rules of the game aimed at reducing injuries, before the season starts, so they can't practice at a distance and limit contact to competitions either. And since football tends to attract a particular sort of coach and parent, there's been little interest in compromising or thinking outside the box to save the season, at least among the powerhouse programs in my area. At least two head coaches in the area are pretty much COVID-deniers (Kids don't get it, it is just a flu, that kind of thing).
I think that's something that's bound to vary from place to place. I'm not worried about injury when it comes to our sports/activities, but the hospitals in my area are pretty much normal. My kids are still doing their usual risky things - skateboarding and biking and such - and the possibility of injury doesn't concern me. The problem with football right now, as I see it, is that in our state the rates of spread vary pretty extremely from county to county. My rural county has a current rate of 14 cases per 100K (weekly), which puts us around 65th in the state right now; the neighboring county where most of our football opponents come from has a current rate of 98 cases per 100K, the 3rd-highest in the state. Why in the world would we want to hold football games that would increase the mixing between our two populations right now?