Why is it considered a disability if you have more finger than normal? you could probably do loads more with these extra fingers and toes, right?

Disability: Extra fingers and toes, besides cosmetic disparity, are not as practical as it sounds. An extra finger or toe can have the same tendons, muscles, bones, nerves as regular fingers, but more commonly, they are less functional because they lack the same anatomy as "normal" fingers and toes. In my experience, most extra toes can have the same skin appearance but lack function and "normal" ability.