You didn't actually explain why. You just mentioned unrelated issues.
'moralising' is actually a pretty good answer. When it comes to certain actions like organ donations, we're utterly terrified of the idea that someone will feel like they 'have' to do it. Getting a million dollars, even though it would have an amazing impact on your life, is bad because you'd feel pressure toward doing it. Getting a small payment would also be bad because you're being taken advantage of. But you can do it for free if you so desire!
'moralising' is actually a pretty good answer. When it comes to certain actions like organ donations, we're utterly terrified of the idea that someone will feel like they 'have' to do it. Getting a million dollars, even though it would have an amazing impact on your life, is bad because you'd feel pressure toward doing it. Getting a small payment would also be bad because you're being taken advantage of. But you can do it for free if you so desire!