There is a very good reason. Engines respond with at most 3 and usually 2 personnel on it. They come from a fixed location and are usually tied up for at most an hour on scene. The ambulance/medics are coming from a staging point but they may or may not be closer to the call. This way there are maybe 4-5 personnel on scene to help with the patient. If all are not needed then the engine goes back in service and can respond to another call. If u use an engine it can be very versatile to respond to either fire or EMS. If u have just EMS responders on an ambulance there r only 2 on a crew. Sometimes that's not enough help. Accidents, cardiac arrests, obese patients are just some of the reasons 2 man crews r insufficient.
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