For whatever reason, misfeeds happen. Hanging chad can flip open and closed. Detached chad can become stuck in the feed path, increasing double feeds and misfeeds. Detached chad can jam two cards together, increasing misfeeds. In some machines, detached chad can jam over the light or sensor, causing holes to not be read until the chad blows out of the way.
Detached chad can migrate from one card to the next.
People cannot count cards as quickly as a card reader, but a card reader is much more limited than a person in how it can handle and read a card. Any damage a card has sustained can confuse a card reader or cause it to malfunction. People are better able to deal with such problems.