As said by Charlie Munger: "show me the incentive and I'll show you the outcome". In the UK, 23% (and climbing) working age adults are now registered as disabled [1]. For a qualifying disability you can claim personal independence payment (PIP) that gets you between £73.90-£110.40 for living plus £29.20-£77.05 for mobility, which is not means tested [2]. That's up to $249.98 USD a week untaxed on top of your regular income - you can imagine why people may be incentivised. Worse still, Citizens Advice which is 60% taxpayer funded [3], actively tell people how to fill out the forms to guarantee a positive outcome.
I have no idea why people would want to register as disabled though... /s
[1] https://www.scope.org.uk/media/disability-facts-figures