[phloxicon]

"But wouldn't any other researcher who obtains the data from doctors who recruited the patients have to give credit to the doctors who shared their data?"

In academia, it's better to be the primary author of a paper. More primary author publications makes it easier to get funding, keep your job and advance your career. If you share the data that you might have spent a lot of time and money collecting, you will either gain a citation or a non-primary author position. This is worse for your career. I don't like the system. Most academics would prefer to share to have their work verified or even to see what others might find but the incentives are to keep it private so that you can squeeze as many primary papers (or papers from your research group) as possible out of a data set before sharing the paper.