Open Question: Why does the Social Security Administration typically deny first claims for Disability as a matter of routine?

I spoke with a state insurance manager with years of experience helping those with pre-existing conditions and she said the Social Security Administration routinely denies first claims for Disability. Indeed, this has spawned an entire section of the legal trade to help those denied Disability coverage so they can get benefits when disabled. In this particular situation, I have a sick relative with Hepatitis and Diabetes who cannot get private insurance due to pre-existing conditions and has applied for Disability under Social Security. They are not hopeful for a response soon -- some applicants have had to wait for YEARS for an answer -- and my relative is now expecting to be denied, just as a matter of the Social Security Dept.'s habit. Why has it become so difficult to receive benefits if the individual is truly disabled? I've heard stories of even paraplegics disabled in horrific car accidents not being able to get Disability coverage through the Social Security Administration. What's the real story here and what is going on?