It's an interesting one, and I suspect one contributing factor could be the coal mining claims.But there are a huge number of disability allowances of one sort or another:
Disability Living Allowance (DLA)
Employment and Support Allowance (ESA)
Attendance Allowance
Access to Work
Disability Premiums (Income Support)
Incapacity Benefit
Work Choice
Independent Living Fund
Blind Person's Allowance
Coal health compensation claims
Disabled Facilities Grants
Disabled Students' Allowances (DSAs)
Help if you have a disabled child
Personal Independence Payment
Reduced Earnings Allowance
Severe Disablement Allowance
Vaccine Damage Payment
Part of the problem has to be that the edifice of benefits has mutated with every new government, so few folk actually know all the benefits that can be claimed. What's needed, of course, is the radical step of dismantling the entire structure completely, and staring again from scratch - not easy to do when you're trying hard to ensure no one in genuine need misses out.
I think the other issue has to be the type of disability and its effects. There are, for example, numerous stages of sight impairment, and it often surprises people to learn that most legally blind people can actually see, albeit to a limited extent. Physical disabilities (legs, backs, etc.) are even harder to assess, since our extremely well-paid GPs can rarely assess such conditions with any degree of accuracy and can, often, become a 'soft touch' for those seeking to register.