Because the patients get little or no exercise.
Muscle atrophy can occur when a limb is immobilized for a long period, leading to loss of muscle mass and strength. This can happen, for example, if a person is in a cast for several weeks after a fracture. Physical therapy and exercise are often needed to rebuild the muscle.
apathy
Muscle atrophy means muscle wasting. It can occur due to long-term disuse or bed rest, or due to neurological damage or malnutrition.When muscle wastes away because of disuse it is called atrophy.
Patients (or animals) who are confined to beds for extended periods of time develop ulcers at the points of contact with the bedding. Friction, impaired circulation & the effects of gravity combine to begin the breakdown of skin tissue. Add in moisture, bacteria... and bed sores are the result. In other word, rotation of the body can counteract some of these forces to prevent and/or delay deterioration of skin & muscle at pressure points.
It depends on how weak the person is and what they are capable of. But if they are strong enough to sit up every 4-6 hours or turn over, things like that will allow the person to slow the progression of atrophy. Whatever it takes to move muscles in the body. Atrophy is caused when the muscles are no longer being used, so the best thing is to use them. Hope that helps, 
The "Bed-confined" definition means: Unable to get up from bed without assistance; and Unable to ambulate; and Unable to sit in a chair or wheelchair
atrophy of tissue
One confined to the bed by sickness., One who receives baptism on a sick bed., A school, or a session of a school or class, in which medicine or surgery is taught by the examination and treatment of patients in the presence of the pupils., Of or pertaining to a bed, especially, a sick bed., Of or pertaining to a clinic, or to the study of disease in the living subject.
Urinary catheterization is employed in hospital and nursing home settings to maintain urine output in patients who are undergoing surgery, or who are confined to the bed and physically unable to use a bedpan.
It is typically a clincal facility where patients are able to arrive for treatment on their own without the need to be transported by ambulance to and from their appointment. The term ambulatory means to be able to move about and not be confined to a bed.
A bed-day is a day during which a person is confined to a bed and in which the patient stays overnight in a hospital
Able to walk around; not confined to bed.