Go ahead, sit down with a piece of paper and try to design it in a way that doesn’t prevent shifting, doesn’t skip under load, doesn’t require you to use a longer chain, works with rear suspension and doesn’t interfere with your legs.

Chain moves side to side with gear changes, and adding extra chain tension with an extra drive sprocket could make the deraileur angry. Also, chain-tube geometry varies a lot. Not impossible, just more complicated than it might seem.

I ride an all-original 1970s steel-frame road bike, which seems like a prime candidate for this sort of product. I'm certain no sprocket-based design would fit it. I'd buy one of these if there was any way to lock it up.