Think about this way, if you argued that the droids don't know what happen to anakin beccause he turned into vader and can't see him in the suit then i would say why don't the droids tell luke who his father is because 3PO was built by anakin and he obviously knows who he is, but since he doesn't tell him that and the person who tells him is obi-wan in ANH, that his father was killed by vader, that would mean obi-wan made it all up to cover prevent Luke or anything from happening to him. It's like obi-wan said " From a certain point of view".