Basically what Hunkadoodle said, everything is an approximation. Shading models get more refined year after year, but they still only approximate reality. It’s an efficiency problem, where you strive for the highest “accuracy vs. time” ratio.
If you look at the old “Cycles volume shader” thread, you’ll see awesome examples of hardsurface models being “carved” out of a box with triplanar-projected textures.
(although I can’t find these specific images at this link)