Having a lot of data is a big incentive to find someone who can analyze it correctly because it will raise the impact of your scientific output. It is not as bad as people describe here. Even large companies need to publish and share in order to show that they know their business and in order to have any shot at a collaboration wit a large hospital (or another company). Such a hospital will also want to publish and share in general. A publication of a company also signals: "Hey come to us, maybe we can work out a deal and provide a nice service based on our work". Yes, there are NDAs, contracts and patents but that company paid a lot of people for their efforts with only a small chance at ROI, it is only fair they are rewarded.

An optimum is often found that favors sharing as little as possible but often "as little as possible" is still quite a lot. Also, how can people label this as unethical when people would not even have gathered the data if they knew beforehand someone would force them into sharing, diminishing the data's return on investment?

I'm also pretty sure the sharing of a complete data set will get you a lot of citations, so there is a financial incentive next to the moral one.