My brother had a bone marrow/stem cell transplant a couple of years ago and I was a matching sibling able to donate. Yes, it's unfortunate that there are so many of us who don't understand how risk free this procedure is to a donor. There are a couple of problems that contribute to this in my opinion.
First, the name of the procedure itself is misleading. It's called a "Bone Marrow Transplant". A lot of times however, there is not even bone marrow taken directly from a donor. I was not even poked in the bone at all. In my case, I was hooked up to an apheresis machine for a few hours, and it basically pumped blood out of one arm, and then pumped back the blood minus stem cells into the other arm. A bag of stem cells is then intravenously given to the patient which then makes its way to the bone cavities and starts growing new bone marrow. The donor goes back home on the same day and hangs out like its any other day.
Second, there are some blood disorders for which they require bone marrow from the donor. Now, again, this is not scary as it sounds. They extracted bone marrow a few times from my brother for tests. There is no "drilling" involved. Yes, its a bigger needle, that they insert into your hip bone after administering local anesthetic. I don't know first hand how much it hurts, but they don't even admit you in the hospital for a day. And for the next couple of days, you are a bit sore from it. My brother has average pain tolerance and I didn't see him writhe in pain at all at any time once the procedure was done (happened in less than 10 minutes). I suspect that a torn ACL, a broken ankle or arm which many people on this list would have experienced are far far much worse.
Third, unlike other types of donations involved in transplant like a kidney or liver, this process doesn't take anything from the donor that is not completely replaceable. If you donate stem cells, they will replenish in your body automatically, and the same with bone marrow. Really, there is no easier way where it costs you almost nothing but average pain and minor discomfort for a few days at the most.
I spent over three months in the hospital with my brother in India when he had the transplant. Fortunately, I was a matching sibling so we didn't have to look in any registry. But I spent my days and evenings with countless families who had a loved one undergoing chemo, while just waiting hopelessly to see if they could find a matching donor in a registry. Few things are as heartbreaking to see as families just not being able to do anything and watch someone in their family just hinge on hope from a database. The least we can do is increase their chances for hope with a cheek swab.