or to comply with the national civil aviation policy.
Some airlines think that passengers will indeed pay more attention to humans waving and smiling to them, so airlines still ask their cabin crews to perform the safety demonstration to make sure all passengers are briefed.
The main reason here is the different types of screens. Most seats will have seatbacks in font of them where they can install screens. That’s not the case in first/business class and in the first row in each zone behind the bulkhead (behind middle lavatories for example). These seats have no seatbacks in front to install fixed screens, so airlines will install the stowable screens, the ones that need to unlatched to popup. This type of screens are not allowed to be used on ground. So if the plane has these kind of screens in some seats these passengers will not be briefed about the safety of the aircraft because the screens are stowed during that time, hence why the cabin crews will do the manual demonstration. Because the international policies require that all passengers onboard revenue flights to be briefed. Also, asking passengers to unlatch the screens before the demonstration and then asking them again to stow it back is not practical. Some airlines install large screens on bulkheads at the beginning of every zone to overcome this issue, so their cabin crews do not perform the manual demonstration.