Reason being that most homebuilts qualify as "Light Sport" aviation... which means you can legally fly it with a light sport license. Otherwise the high blood pressure medication that your doc prescribed (so you can watch the nightly news without popping a vein whenever 0bama appears on the screen), means automatic revocation of your regular private pilot's license because you can't pass the medical.
I know. My mom’s husband has one of each type of plane and recently went on BP meds. He can only fly one of them right now.