* more frequently sent to rural homes, where there might not be an available hydrant, so they need to transport all the water they're likely to need
* same for wildfires
* American homes more likely to be wood+drywall
Drywall is non-combustible and used as the main component in many fire-rated wall assemblies.
You've also got a lot of volunteer departments where each firefighter keeps their equipment in their own personal vehicle.
Why firetrucks in the US are so large is a good question. In the spirit of Chesterton's Fence, I'd assume that there is a good reason until proven otherwise.
This is a really simple fix that the rest of the world does better.
This article has some nice points of comparison between typical American firetrucks and a Japanese firetruck: https://www.sfchronicle.com/local/article/Meet-Kiri-the-tiny...
Lots of safety regulations stem from that divide. Wood is a bit more flammable.
(I'm not saying the difference in safety regulations still make sense today. I'm talking about one of the historical origins of the divide.)