There is a question for which i do notseem to get an answer everywhere. Consider this...
Someone lives a normal life until one day, an accident happens and the person severely injures his brain and then becomes a 'vegetable'as they say....no thoughts, joy,anger...complete loss of any thought whatsoever....can you see any purpose in that???
Try to see a bigger picture. Life in a physical body is only a part of our Conscious Existence.
I am sure that you will agree that learning to die (learning to separate your consciousness from your physical body) is a very important part of our lesson of conscious existence in the Universe.
Yogis and Lamas in Tibet used to spend lifetimes practising extreme exercises to understand the connection between the body and our Consciousness. Switching off many functions of the body, due to accident or any serious disease is just another way.
Brain damage just limits a degree of control over the physical body, but I can assure you that people who are severely impaired, in a coma or even in the state of clinical death, have feelings no less intense and precise that you have when you dream. The ESSENCE of the individual consciousness has very little to do with the physical body.
Also, we learn MUCH more from our suffering and our limitations than when we are living a "normal life" as you call it. Basically, before you are born - you can choose either to die quickly 100 times, die very slowly once and anything in between.
Also, search This Forum for "retarded" and for "poverty" to discover the likely reasons of being born retarded or in poverty.