It's a manufacturing limitation. Integrated circuits are nearly universally built via photolithography [1], which only makes flat 2D layers. Stacking many, many

It's a manufacturing limitation. Integrated circuits are nearly universally built via photolithography [1], which only makes flat 2D layers. Stacking many, many layers for 3D circuits is feasible; building layers on different sides of an object would require re-engineering every part of the process, including the use of large silicon wafers, for a much more complex process with negligible (if any) benefit. [1] https://en.wikipedia.org/wiki/Photolithography#Basic_procedure https://en.wikipedia.org/wiki/Photolithography#Basic_procedu... edit: kragormonkey beat me to it!