They can't patch the OS because some applications rely on the UDID being constant. For example, some games check if the scores/progress are real by storing a ha

They can't patch the OS because some applications rely on the UDID being constant. For example, some games check if the scores/progress are real by storing a hash of the scores and the UDID. This allows the dev to check that the scores/progress file wasn't downloaded from internet. Changing the UDID would make the application think that the scores were tampered with.