[unfletch]

(Without any real evidence) I think this usually happens intentionally in new construction. It's actually to the homeowner's advantage to place their driveway such that cars can't park next to it:

In SF, having a driveway with a bit of unusable space on one side gives you an extra parking space -- on the street. Blocking your own driveway there is legal. Even if you have room for your car(s) in a garage, I'd imagine an extra space is frequently useful in a city with such limited parking.

A bit of dead space also provides a buffer to turn into and out of the driveway that might sometimes be necessary (your turning radius isn't zero and some streets are narrow).

(Background: I lived in SF for 12 years but never owned a car or driveway there.)

[astrodust]

It's a feature, not a bug, not having a parking spot in front of your house.