It's just one of the oddities of the English language. The term "parkway" derives from a blending of "car park" (as in "parking garage") and "railway," obviously meant to invoke the feeling of driving like a train down a path, but in a car. Why English chose to take the "park" part instead of the "car," who knows. "Driveway" simply comes from blending the words "drive" and "way," meaning "a stretch of road to drive on," but again, why English speakers chose two words that would convey a feeling of continuing to "drive" down the "way" and not chose something like "parkway" instead, is again, just an oddity of the evolution of the English language. Consider how arbitrary the word "dog" is, why does that sound convey the meaning of a four-legged hairy man's best friend (as opposed to "bark," which actually sort of sounds like his word of choice)? It doesn't, but all English speakers have accepted that that's what that means, and we all use it knowing that we all know what it means - a collective agreement, if you will, to use certain words to mean certain things. So while "parkway" and "driveway" don't make sense and should probably be swapped, English speakers continue to use these words with the understanding that everyone else knows what they mean, and that a swapping of the two previously-established meanings would confuse everyone.

Originally parkway was used for a railway with a carpark at the edge of town, according to http://en.wiktionary.org/wiki/parkway . Driveway, according to http://en.wiktionary.org/wiki/driveway comes from drive + way, which makes sense as it is a short private road you use to drive toward a house or garage.

There are many other similar oddities in how the English language (and probably any other natural language) developed. Some examples:

- A pineapple is not an apple and does not grow on a pine tree.
- A hot dog is not made of dog meat (thankfully!).
- A grapefruit is a fruit but has nothing to do with grapes.
- We say our nose runs, but our feet smell.
- When a house burns up we say it burned down.
- When a pitcher winds up he's getting ready to throw the ball, but when a writer winds up he's getting ready to stop.
Etc. etc.