There are a few reasons why we don't do more layers, they mostly boil down to logistics and providing value.
Firstly, when we cut a sheet of material it produces 200pcs, a lot of layers just aren't/wouldn't be that popular. It would be ridiculously time consuming to cut individuals or partials ramping the cost of a single layer to point where it would be insane.
Secondly, every different product/SKU we add requires effort in photography, description, pricing, stocking, shelf space, etc. This means we always have to consider the value of adding a new SKU to our inventory.
Thirdly, we make a laser cut case and we'd hope that people would reach out to their local hackspace or whatever to help them produce custom layers as they need them. There are online services for doing custom cuts too (when you see their pricing you'll understand why short volume runs become somewhat infeasible).
Fourthly, shipping a single slice of delicate acrylic is a nightmare. We actually laser cut a card wrap for them adding more material costs, laser time, and packing time. We only offer it at all because we want people to be able to mod their Pibow - but honestly it's a total pain.
Ultimately we try to offer a good range of options with reasonable flexibility at sensible price points. It's just not possible for us to create every different possible option unfortunately!