If memory serves me correctly the firemen at the airport I'm familiar with do not sit around kitted up, much like the civvies they dress for action on the run. Re ambulances its probably cost, I can think of only two fatal accidents on site in the 14 years I've been on my airfield, southern uk. From what I know both were unsurviveable, so they wouldn't have been cost effective. Hopefully we get a heads up on an incident, i.e. mayday or pan, which allows 'civilian' services time to respond. Having said that both above accidents had no warning. Under the Aerodrome Manual there are criteria laid out for RFS services, but not as I reacall specifically for ambulance or paramedics.