But I have three spark plug sockets (two different sizes) in the garage, which happens to be about 75m away from my house. I would walk there to get them.

Remember, the question is: “I need to replace a spark plug. The garage is 200 meters away should I walk or drive there”

The answer is absolutely "walk", in many cases.

If the question had been "I need to take my car to a repair shop for some work. The shop is 200m away. Should I walk or drive?", then the answer would be less ambiguous.

I'm guessing my car is old enough that is comes with a spark plug socket in the toolbag in the back along with the jack and spare wheel; you're right it probably isn't standard equipment anymore. (Car is Mazda from 2005 for reference)