I'm curious. What's the reasoning behind the idea that cleats need to have a fixed position? (below the ball of the foot presumably?) Other than it's always been done like that?

The way I see it pushing cleats forward serves no purpose whatsoever. Pushing them back however means you need less calf muscle which is an unnecessary muscle for cycling in any case. Less energy use, more foot stability which translates to better knee stability and onwards goes the chain.