I think it is just regulatory inertia. Those of us who were flying in the '70s and '80s will remember a different relationship between company accountants and pilots, and Captains were closer to God in those days. The accounting wasn't so tight and the senior pilots of the day had been through the '50s and '60s where accidents and engine failures were more common, and the tombstone imperative had moved not so far along its track. The suggestion that operating with less than full thrust would save the company money was not universally well received, nor were concepts like intersection take-offs. The regulators, probably of the same generation, seem to have agreed and imposed a series of caveats on assumed temp take-offs such as 'not on contaminated runways', 'not with anti-skid or reverse inop', 'not when wind shear is forecast after take-off' etc., and, once written, it takes a brave regulator to remove the caveats. Derates were rare. On the L1011, for instance, nearly everyone had RB211-524B4 engines but just by chance one airline, BWIA, had ordered the the jets with RB211-22B engines, a lower rated thrust. That meant we could manually set the EPR for the derated thrust, accompanied by severe warnings about selecting full TOGA thrust in emergency and great oaths not to do so in the take-off brief - no automatic protection then. The argument that a derated take-off was a normal take-off was accepted at the time, and it was permitted by the regulators without caveat. Over time modern engines became available with many more derate settings now selectable from the FMS. I can see no reason in principle why a reduced thrust / SEL OAT / FLEX / Assumed temperature should not be also be treated as a normal take-off. I note that the AMCs to EU OPS still make it clear that reduced take-offs are always at Captain's discretion.