One explanation is that quantifying the distance depends on the size, scale, mfr, and model, since not all units would require the same clearance.
For instance, a realistic amount of clear space needed around a multi-MVA transformer for effective cooling, is a lot different than the clearance of a 1kVA transformer that you can lift with just one hand. This is a clear space that is better suited for the manufacturer to advise upon, than for the NEC to have a one-size-fits-all dimension, since all transformers have different thermal footprints and cooling methods.