It's because a driver can do everything right and still miss a cyclist. Humans aren't infallible, and driving is an extremely complex task. In many cases it's an issue of bad road design or cyclists not following the rules of the road. For a driver to be held criminally responsible they have to be negligent, eg. using a cellphone, speeding, distracted, etc.

From my own experience interacting with cyclists on the road, the riskiest situations I saw were ones the cyclist created. Eg. Running reds, doing 15+ mph on the sidewalk, lane-splitting, etc.