Although exactly what Goofy is has never really been established, it at least seems obvious what Goofy isn't - he definitely isn't just a normal dog. He kind of looks like a dog, in that he has a dog-like head; but dogs aren't usually known for standing upright and walking on two legs.
After giving it not a lot of thought at all, I've arrived at the fairly obvious conclusion that Goofy is some sort of dog/human hybrid; possibly the result of experimental genetic tinkering. He's able to do many things that people can do, such as walk and talk, wear clothes, and yes, even drive a car. Pluto, however, can't do any of those things. Why? Is Pluto retarded or something? Well, no... not really. I mean, compared to a human, Pluto possesses roughly the same mental acuity as a retarded infant with severe brain damage and an extra helping of mongoloidism, and the cranial capacity of a deformed troglodyte with a microcephalic head and a rare case of double downs syndrome. Compared to another dog however, Pluto is just a regular dog. Maybe even a doggie Einstein... but a regular dog, nonetheless.
However, for the sake of a non-biased exposition, it must be noted that Goofy is a fantastically stupid creature, despite his human-like characteristics. He also displays an alarming lack of coordination which, considering his abominable nature, shouldn't really be that surprising; seeing how Goofy is basically the result of the tortured marriage of animal and human genetic material, haphazardly spliced together, then forced backwards through a French milking machine and thrown at the wall to see what sticks.
But I digress.
So, after ruminating not very deeply on the subject for not a very long time, I've concluded that Goofy's innate clumsiness is due to the basic incompatibility between a canine nervous system and a human musculature. The resulting slapstick is that of a body which is constantly in the throes of raping itself.
In conclusion, the point of this long winded discourse is basically this: The reason why Goofy can drive a car and Pluto can't is because Pluto is just a regular dog, while Goofy is a horrid abomination of nature, a walking blasphemy and a Testament for all that is unholy; a putrid and vile discharge from the womb of evil... the offspring of Satan, the Antichrist embodied in the cancerous flesh of a thriving tumor and wrapped in the mortal coil of a three headed serpent.
I suppose, however, that some transcription errors are to be expected in such an unwieldly attempt at genetic manipulation, so it would be wrong to place all of the blame solely on Goofy, just for naturally being evil. It's obvious that the fault clearly lies with the Satanic, devil worshiping Disney artists and animators.