As cynicalkane said, you say this because you haven't met people who are very serious affected by these illnesses. These people will greatly disagree with you:

As cynicalkane said, you say this because you haven't met people who are very serious affected by these illnesses. These people will greatly disagree with you: yes, they are interesting and clever, but they do _suffer_ from their afflictions. Saying that these conditions do not exist discourages those individuals from seeking diagnosis and accommodations for their conditions. For example, an individual with ADD [1] might be very intelligent but will be unable to focus on timed tests and will suffer from worse academic performance. If they're unaware of their ADD diagnosis they and others around them will think they are lazy and/or stupid; this becomes a self fulfilling prophecy. Being diagnosed with ADD doesn't just mean a prescription for medicine, it can instead mean being allowed to take more time on a test or to take the test in a closed office. This can mean they have a chance to be admitted to a university and be exposed to intellectually stimulating material (whether it's computer science, physics, economics, or philosophy).