It's a good question. Many of those who apply for benefits from the Social Security Disability Insurance program may feel as if the complexity is designed simply to make their lives more difficult and that the process is so complicated that the goal must be to prevent applicants from obtaining benefits despite their suffering a disabling impairment.
The complexity is in part due to the breadth of the types of conditions that could cause impairments so significant as to leave one unable to successfully remain in the workforce. Some idea of that complexity is visible in the comments to a recent rulemaking from the Social Security Administration (SSA). When SSA adds or revises a physical or mental condition to the materials that claims' evaluators will use when determining an applicant's eligibility for benefits, they must do so with precision.
The recent rulemaking, entitled "Revised Medical Criteria for Evaluating Mental Disorders" announces the final rule which goes into effect in January of 2017. The notice explains the proposed rule was submitted in 2010. When a proposed rule is announced, the agency must collect comments from the public, which can then affect the final rules. For the revisions to these rules on mental disorders, the SSA received 2,245 public comments.
The agency must go through those comments one by one and evaluate the comments. In this notice, they list 16 densely-typed pages of comments along with the agency's response. In many cases, they note they did make changes to the language in the final rule based on the public comment.
Yes, the process is slow; the first iteration of this set of rules was published in 2003, so just this work revising the rules pertaining to the evaluation of mental disorders took 14 years. When people complain the government is not responsive, you have to remember to be careful what you wish for, as every step that makes the process more "responsive," also makes it slower.