A separate neutral has been required for a range/dryer outlets for something like 25 years. When electric ranges and dryers first became common in the 1950s, they allowed the ground to double as the neutral, probably for cost reasons and so that people could just use SE cable for the range (shudder). That was before grounded outlets too, so I suppose they hadn't thought out the whole ground/neutral thing yet either. Myself, I tear out any 3 wire range outlets I see, if only to get rid of the 60+ year old SE cable with actual rubber RW conductors.
Why it took so long to require 4 wires doesn't make much sense to me either, other than I suppose the risks are pretty low for something bad happening. You don't hear a lot of stories about people being zapped by dryers or ranges.
On a side note, I worked at a water research lab built in the 1930s that still had a lot of ungrounded 3 phase outlets as of the 1990s. We had a fair amount of equipment that had alligator clips hooked to the ground wire so we could clip to the metal boxes. Kind of frightening to think about, ungrounded 3 phase outlets in a water lab. Actually one off the more terrifying things we found when they finally rewired in the mid 90s, was that that there was no neutral line between two floors of the building. The entire neutral load/unbalance was carried through the conduit.