Hey Goshawk; there might some that "game" the system," however I can assure you first hand there are "controls" in place.
My application for disability was more then 100 pages. They check your work and medical history. You give up every ounce of privacy with regard to your health and medical records. You must document and account for your activities for ever hour of the day. Family members are also required to provide statements. To this day I have no idea what my doctors and family said in the the application process. I also was examined by two Social Security Administration Doctors that are trained to spot cheaters. My process reached back more then five decades. This all takes months, some wait years, all while not able to work or support themselves while workers at the SSA pour over hundreds of pages for a single case. Yes, sometimes those workers are short with you, but put yourself in their place, handling thousands of desperate people who have no way to support themselves and in many cases manage their medical condition. Some don't have the capacity to complete the process and simply drop off the radar. If, and once, you are approve; you are subject to review the entire process at anytime. Yes, there are "controls" in place, all this for an income that is far below minimum wage in any state. And yes, employers, if they would follow the law, we may have less people on disability.
It is far, far more likely that there are many more that should be on disability that con not qualify because the system is so rigorous.