Because the generalization changes the question. There is a world between "can drive 90 % of the time on 90% of roads" and "can drive 100% of the time on 100% of roads." The former is still extraordinarily valuable, the latter effectively impossible. When you conflate the two there really isn't even a point in having a discussion.

In the given example "driving in a parking lot +rain" it's completely reasonable to pass the buck to the human driver. In your example "driving +rain" you can't because that situation occurs well more frequently.