One simple reason might be this over: people make errors over time, but they become familiar with the case. That familiarity is lost whenever they hand over a patient. And you can't solve that with documentation since people would be busy all day doing paperwork instead of caring for patients.

This problem exist in every domain, and in medicine it can have deadly consequences. People sign up to become doctors knowing that they will have to work crazy shifts. But I guess they underestimate how the system hobbles them since it is focused on being efficient, not on achieving the best outcome for the patients.