Because scripts involved in the actual rendering or ray tracing process would be uselessly slow.

The ability to qualify which polygons react to which lights, and which shadows, is actually a non-trivial change to the renderer, given how it is built. (This info I got from stewer.)

Far more possible would be material room changes that offer variations of the lighting nodes with more options. For example, a Diffuse node that has a specific list of lights it consults, and whether or not to pay attention to shadows would take care of it.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)