Why are one way flights more expensive?

The reason one-way flights are more expensive than return or round trips is because in the case of latter, you are letting the airline know that you are going to use the same airline for your return trip, and not with any other airline.   When you take just one-way airline ticket, it is bound to be expensive because there is no guarantee that you will take the same airline.   So making round trips cheaper than one-way flights are a way of revenue protection.

Airline companies want to stretch their profits to the optimum with one-way ticket,  while on a round-trip, the profit margin they earn on the trip, is much lower.

Airlines also assume that one-way trips are often purchased by business travelers, which is why they put up high prices.  Business travelers working with large companies have tie-ups with the airlines, due to which they can benefit from bulk discounts.  But they also offer better incentive in the form of cheaper prices for round trips.  But we are also seeing a phenomenon where many round trip tickets cost the same as two one-way tickets, especially in domestic US flights.

 There are airlines that price their round-trips in the same manner as one-way flights, subject to the availability, examples being Virgin America, SouthWest, JetBlue and Amerian Airlines. Another point to note is that one-way airfares are often refundable as they are  less limiting in nature.

You will find the methodology of one-way flight tickets being more expensive than round-trips, usually in international tickets.  The round-trip fare is like a reward for advance purchase.  Domestic US flight tickets or short-haul flights in Asia or inside Europe are more expensive than advance booked round-trips.

Airlines with promotional fares offer discounts on one-direction substantially, for instance Norwegian and WOW Air.  For instance, there are flights to Europe for just $150 and $200 but the return trip may cost $300 to $400. In that case, you can opt for another carrier or use airmiles for the return.

So one possible benefit of booking one-way airline tickets is that you can get cheaper or best options for each aspect independently, rather than limiting yourself to one-career on both ends.  This may also include using different airports for arrivals and returns, when applicable.