The reason why airlines have this policy of canceling the roundtrip if you miss the outbound flight is because it used to be (maybe it still is) that you'd get

The reason why airlines have this policy of canceling the roundtrip if you miss the outbound flight is because it used to be (maybe it still is) that you'd get a lower cost rate if your itinerary included a Saturday night stay (weekend fare). If you booked the roundtrip to get advantage of the lower fare, but only intended to use the return segment(to travel on a Monday), then you'd get a lower price than if you booked a one-way on Monday. Also, people would start the ticket from a low-cost city hub so they can take advantage of the cheap return fare. Pricing probably doesn't work that way anymore, but the policy still exists. Any seasoned traveler should already have experience with this. To avoid the problem, you can call the airlines BEFORE you miss your outbound flight and they will provide options for you if you are nice.