Because the point of a learning disability is that the person is working their butt off already but still struggling.
It's like having two people try to catch a pig, but one of those pigs is greased. If you tell both participants to use exactly the same effort to catch the pig, the person who has to get the greased pig will probably fail. The person with the greased pig may even try harder than the other person yet still fail to catch the pig.
There isn't an easy solution to mental illness or learning disorders. Again, I am not saying that we should tell people to not try hard, I am just saying that people shouldn't go around saying learning disabilities are an excuse for the lazy or those who don't like to work their butts off.