One reason (at least) with two parts: US cars are too large, and fire departments in the US cover massive areas.

American care are huge, so fire services rely on their trucks to clear obstructions in an accident. In asia/europe, manual clearance is still the norm, but you just can't do that with ex: a rolled F350 that's blocking stretcher access down a two lane road. And given how remote a call might be, every second matters because its 25 minutes by air to the nearest hospital.

The other aspect is that huge coverage areas mean any given call could be in places of deep urbanization or the most remote rural bastions of podunkia. The trucks need to be able to go offroad, carry enough material to address emergencies entirely unsupported (big water tanks) and bash their way down a glorified ATV track. Dispatch areas are too large to have specialized FDs, so city roads have to be big enough to support trucks that can do both.

In short, we have to have big trucks because we won't fund enough fire departments to allow them to specialize to a specific region's geography.