It's a competitive thing. You generally see one way domestic tickets for half the price of comparable roundtrips in markets where one or more budget carriers (such as Southwest) offers one way fares.

For international, the one way fares are generally purchased by those relocating overseas, where either they must take the trip or the employer is paying, or by people with complicated multi-city itineraries who cannot qualify for RTW and similar fares, where again these are very likely to be business travelers who will be reimbursed by employers or clients. It's a matter of market segmentation and price discrimination.