Kernel development is largely an optimization process of the internal model of what a kernel should do, and for that developers need first-class raw data, the DRM maintainer wants to know how you'd like to change DRM. And though you can put that into a translation/abstraction layer, it's not helpful, because that doesn't scale, because the maintainer would have to look at every such layers and come up with a common one, and repeat this grueling task every time they want to move forard with DRM itself.