That’s essentially what they do already. If you look at the photo The defenders offside line is drawn from his knee

I would ask you why you think track running isn’t measured by the runner’s feet then as well. Why does a runner cross a finish line when they cross with their head and shoulders?

If you look closely The defenders offside line is drawn from his knee a playable part of his body

Are you implying you can only score using your feet? Because what if the person is offsides in the Box and they win a header for a goal but their feet were behind the defenders like in this photo. It’s still offside because you can score with any part of your body, aside from your hand and arm.