I suppose we'll never know why, but this wonderfully Dickensian picture answers when: as soon as humans invented the first self-propelled machines, dogs were proudly riding shotgun.
[Couldn't get more perfectly Dickensian! From left: J. Marley, B. Cratchit, E. Scrooge, T.T. Cratchit.]
The current icon shows Polistra using a Personal Equation Machine.