Why can't I buy a one-way Saver reward with my Miles?

Our Saver rewards are specially discounted tickets with certain restrictions, such as maximum stay, which means these fares can only be offered on a return basis.

Related questions

What are the differences between 'Skywards Flex' and 'Skywards Saver' fares?
Can I book flight rewards and upgrades online?
Why should I buy a Skywards Flex fare when a cheaper Skywards Saver fare is available?