because if you gave them a test that tests for MR they would fail.
Education is part of "normal development" and as you get older, it becomes harder and harder to learn things like reading if you don't start to learn how to do that at a normal age- say 3-6- then it puts you behind the curve. It becomes harder for the brain to learn abstract concepts as we age. That's why educational systems are set up the way that they are. So, while it is possible to learn how to read at say, age 21 for example, it would still be a lot harder. It's sad when someone in say, India doesn't have the opportunity to learn. But believe it or not, a lot of times MR can be prevented. The environment has a lot to do with it- as in home situation.
Also, because you are MR, it doesn't mean that you can't function in society.