For trains, where 1 person directs 200+ cars, the reward just isn't great enough. For trucks, where one person directs 1 truckload, it's 200+ times as rewarding.
Plus, almost every country in the world has stopped building infrastructure to allow trains to go where you'd want them to go. So trains are only useful for huge manufacturing firms and things like harbors. Goods trains don't even go to most airports anymore.
Bit of an aside, but I've often heard it argued that trains make more sense for passenger traffic.
The reasoning is that you can shift a lot of people into one central station, and they then deliver themselves over the last stage of a journey through ambulation.
If downtown Palo Alto near the station were full of skyscrapers, as its land value would justify, the Caltrain would be an effective commute for many more workers.
Plus, almost every country in the world has stopped building infrastructure to allow trains to go where you'd want them to go. So trains are only useful for huge manufacturing firms and things like harbors. Goods trains don't even go to most airports anymore.