Round trip flights have, since almost the beginning of the airline industry, been cheaper than twice the one way fare.
Most common in the years prior to the 1960’s was a 10% discount on the round trip (and there was only 1 fare per class/per route, not a dozen and a half!).
Another great thing that fares use to have was free stopovers. If you were flying, for example, from SFO to JFK via STL, you could stop off at STL for a few days and then continue your journey with no additional fare charged. This great practice seems to have gone away beginning in the latter part of the 1960’s.