I thought about this too. I'm not a doctor (but I do have a degree in Exercise Science), I have doubts that PRP would have much of an impact if there is not an existing injury. PRP injections stimulate the healing process, but if there is no injury or no inflammation then perhaps there is nothing to trigger any response?
Now, UCL injuries can start off very small and gradually turn into a major tear over time, and when it first starts can go unnoticed. If pitchers had imaging done on their arm on a recurring basis as a precautionary measure, then I guess they could catch those cases where the damage to the ligament is so minor the pitcher doesn't even feel any pain, and try PRP therapy to see if the ligament can heal.