Why can fitness to drive be impaired even by just a small quantity of alcohol?

You answered correctly. Even small amounts of alcohol can negatively affect your ability to drive. Alcohol reduces your field of vision, so you may not notice hazards at the edge of your sight. It also makes it harder to judge distances and spaces, because spatial vision is affected. Both effects increase the risk in traffic, even if you feel only slightly impaired. That is why the law considers any amount of alcohol as potentially dangerous behind the wheel.