I could sit here and list out all the medical and legally accepted reasons that the Veterans Administration, and as a result all other regulatory bodies, give me this status (epilepsy, constantly dislocating shoulders, hip arthritis, tinnitus, PTSD etc...)
However I have friends, colleagues and neighbors who are measurably less physically and mentally capable but are not considered disabled.
When I look at the job market, MOST people are not capable of doing their individual life without significant training, help, direction or supervision. People call this bad-state "adulting" and find it onerous.
Said another way, the vast majority of people do not have the cognitive ability, education (self taught or formal), or experience to set goals, take actions and self improve without significant direction from someone else. I personally consider that disabled.
So if these people are considering themselves disabled, then they are probably right - they are incapable of being independent or living within a community that takes care of them. So the only remaining community left is the abstracted transactional monetary system implemented by governments to allocate resources toward them provided they check the right boxes.
Ideally 100% of humanity would be on disability - because we can build systems measurably more capable than human labor - someone just needs to rebrand it as "UBI."