For normal, fully-flexible fares, roundtrip is either double one-way or there is a small discount to encourage two-way business and because there are some economies of scale - the two journeys require only a single booking transaction, a single ticket, etc.
These fares are normally aimed at business travellers who are not price-sensitive but value the flexibility of being able to change their booking or get a full refund if they cancel.
Cheap return fares are aimed at filling otherwise empty seats and carry conditions to discourage business travellers, advance booking and in particular the need to have a Saturday night between the outbound and return journey.
Since such fares only need to cover the marginal costs of carrying one more passenger, plus a bit of a contribution to the overheads, they can be very cheap and still make economic sense.