Rapid iteration/advancement of hardware isn't quite here yet, but we do have reason to believe it's possible (cf. additive manufacturing/3D printing). We can pump out software quickly because we've removed a lot of the bottlenecks for doing so (e.g. using IDEs with keyboards and mice rather than punch cards or manually toggling switches) and made the ability to code widely available (allowing for massively collaborative software projects).

Hardware has a pipeline, and that pipeline is pretty constricted at the moment in comparison to software, but that doesn't mean it will be that way forever.