This really isn’t so hard to understand, is it?
Imagine you’re the train company and have all this empty space with tens of thousands of eyeballs staring at it every day.
Just like any business, the Japanese owners of these train companies aren’t going to overlook opportunities to make some extra profits.
Plus it’s more interesting to look at than empty space.
I like to practice my Japanese reading from the kana in the ads. I know my wife gets really tired of me exclaiming “Hey, I can read this one!”
Besides, sometimes there are some bizarre ads to keep you entertained too.