Why is it people ume that people with disabilities are incompetent, but criminals are competent?

because criminals need to be held accountable for their actions. if they can stoop down below that level of normal, then they should and will be held accountable for their crimes. people with disabilities, as you are implying mental above have a genuine health problem. they are not viewed as "stupid" or "dumb". they simply are given more leniency and empathy. Its a good thing. and most people with disabilities dont go around committing heinous crimes. its just rare.