Back one million years ago when I was playing, we had a playoff game where it was cold as is usual in MN at the end of the football season and an opponent of ours suffered a neck injury. We were in the boonies where several towns had co-opted together to form a school and field a team. No idea where the ambulance had to come from, but we had a 45 minute delay. IT WAS AWFUL. Kid laid on that field with coats and blankets and whatever they could find piled on him.
Having said that, I don't recall that I've officiated a varsity game in the last 23 years without one on site. Hope I don't have to anytime soon either. But in these smaller communities many have volunteer on-call ambulance services and if there is an ambulance call or two they may not have the resources to stay. We also never have them for lower level games. And that can be bad. I've worked a couple of those where we've had to call the ambulance to come.