Car size is proportional to car owner's stupidity. The dumber you are, the fancier the idea of mindless communication on the road is for you. And stupid people want their message to be the biggest around, but you can't do that while writing normally, so, you get what you get.
I think it is more that the appeal of forms of communication where you can exclaim things without having to worry about pesky things like responses increases the dumber the things you want to say are.