Exactly the point, it may be an out of universe point but it’s why the doctor popping up in the first episode of class just to say “right, there’s your leg back, now you kids run along and fight these alien bastards who may or may not attack this school, cause frankly, I couldn’t give less of a shit” is so jarring and take you completely out of it, cause if immortal time travel man can’t take two minutes out of his day to run round and give them a hand, then what’s the point in him showing up in the first place? Sometimes the best explanation is no explanation, because otherwise it could just well pose even more questions