If you are defending your home against an invader, taking prisoners is counterproductive. They have to be guarded, fed and housed somehow, at a cost of manpower and supplies. So you kill the bastards.
If you do that then soon the invaders will realize it and start killing the prisoners they took and the defender will react to that and so forth.
Soon you have total barbarity with no quarter given on either side, perhaps going to Genocidal attacks on civilians. Geneva connections were intended to bring some degree of humanity to this particularly beastly human activity.
Personally I think the conventions are a good idea.
Ukraine welcomes prisoners and that is not what is happening in the video, they are shooting and moving feet away from pop-out targets.