The problem is that the layers content needs to be different, it cannot be shared:
i.e. on level 1 there are mountains on the background layer and on level 2 there are clouds.
But you still want a shared layer setup that is consisten across levels, like [postProcessing, foreground1, foreground 2, world, player, enemies, background1, background 2]. Now after creating 100 levels and noticing you need to make some change to that layer set-up you need to repeat it 100 times.