Don't forget that you're not buying the same thing when you get an expensive one-way fare. You're usually getting an unrestricted fully flexible ticket, or something close to it.
I think that the traditional reason that airlines didn't sell cheap one-ways was that it would enable pax to circumvent the restrictions on the cheap return tickets that effected the market segmentation to make sure that business travellers (who had different travel patterns) would have to pay the high fares that they were actually prepared to pay - although we'd all naturally prefer to pay less - while leisure travellers who were not prepared to pay those fares could benefit from the cheaper fares on offer.
But there are carriers that will now sell cheap one-way fares - bmi for example. The fact that different carriers do different things is all part of the great experiment in which we're living, in which the major airlines try to see whether and how best they can compete with the low-fare airlines.