You can see a bodypart sticking out because there is a contrast between the player and the background and in theory the part being offside should always be visible because it’s closest to the goal. How do you see what hip is closer when there are other players‘ legs in the way? Just imagine a free kick into the box with half the teams standing on one line. This is hard enough to call as is but how are they supposed to see everyone’s hip position?

So you would need VAR for pretty much every offside call? There is no way a ref can call precise center of gravity on their own. How would you enforce that rule in lower leagues or even amateur play?