I know the reason, and it’s a doozy. Send me $5 and I’ll let you in on the secret.
Would you feel the same if they got an "experimental treatment" from some quack who ended up giving them a highly dangerous chemical which resulted in a slow and extremely painful death? Because that is 100% what is going to happen when you remove all regulations for experimental medicine.
Similarly, you already cannot buy all cars privately anymore. Business-to-consumer is highly regulated (that's what the Federal Motor Vehicle Safety Standards are for, but home-built cars also need to pass several inspections before they are allowed on the road.
"Sorry, you can't buy cars privately any more -- you need a gov't agency to make sure you don't get screwed. You cannot fend for yourself, right?"