Except most people also use trucks as daily driver vehicles. You can't exactly fit the wife and kids in a kei. Sure you could also own a car for that but now I need to own/store 2 vehicles instead of one.
Truck works well for those role because it can do so much. It isn't the best for most of those, but it can do them.
Jokes aside I could purchase a new hatchback and a small old Kei truck for a fraction of the cost of something like an f150
Quieter than the other way around.