An MVNO business is low margin (relative to iPhone margins) and requires a lot of marketing to become effective. If Apple were to launch an MVNO, they would have to massively increase their marketing budget for a low margin service, while competing directly with carriers that sell their phones AND provide their wholesale network access. Keep in mind average revenue per user is declining for wireless carriers and MVNOs. Apple has no interest in investing in a declining martgin service that hurts its ability to maximize phone sales.
It's not an attractive business model for Apple, while it may be for Google. Google doesn't make hardware (not even Nexus devices), while Apple does - quite profitably. Apple depends on carriers to sell that hardware for profitability, while Google wants to increase software (Android) and search usage, and low cost network access does just that.