One-Way Ticket

Why are one-way plane tickets so expensive as compared with round-trip tickets? I got a round-trip ticket to Europe for just $50 more than my one-way ticket. What if there were a service that matched two people taking one-way tickets in the opposite directions and just had them split a round-trip ticket? It would save money for the people, and the service could easily take a cut.

[It doesn't exist because of security. The person who buys the ticket has to be the one to present ID.]