There are several reasons for this, including the inability of healthcare institutions to constantly monitor post-operative deterioration as well as patient's inability to visit the healthcare center for post-operation followups. Most of the patient data is recorded manually. This data is often weaned from displays of multiple monitoring devices like vitals monitor, infusion pumps, and ventilators. More often than not, these devices do not talk to each other or to a central system.
Doctors, nurses, or other care-givers manually note down patient statistics in their files and notepads. Even though the healthcare industry is in the throes of digitization, there's still a lot of paper doing the rounds. eventually, the data from these physical sheets might be fed into an electronic system, but there is a high chance of error in that process.