Valid question, and I hear it all the time. Most of the time it's due to preparedness and staffing. By having those 4 people on a fully equipped engine, if something big (structure fire, vehicle extrication, rescue) happens, they can jump in and go with a vehicle full of tools. (provided the ambulance crew can take over).
Otherwise if they're in a car, they'd have to drive back through traffic to the station, move their gear to the new vehicle, and drive back to the scene. It can cost valuable time. Fire engines carry a surprisingly large amount of tools and equipment for a variety of purposes.
That being said, many larger departments are trying out "cars" (usually an SUV) with two people and a med bag to go to medical calls. While the engine/truck and crew stay at the station. This is fairly expensive with the new vehicle, equipment and extra staffing. However it is being done now with success in urban areas.
Otherwise if they're in a car, they'd have to drive back through traffic to the station, move their gear to the new vehicle, and drive back to the scene. It can cost valuable time. Fire engines carry a surprisingly large amount of tools and equipment for a variety of purposes.
That being said, many larger departments are trying out "cars" (usually an SUV) with two people and a med bag to go to medical calls. While the engine/truck and crew stay at the station. This is fairly expensive with the new vehicle, equipment and extra staffing. However it is being done now with success in urban areas.