You talk about the 'extra costs [that] being disabled brings' and people envisage the lame, the crippled, or those with profound congential health issues that fundamentally reduces their ability to navigate the world independently.
In practise, we're now in the business of granting PIP for ADHD, alcoholism, anxiety, depression, Tourette's, urinary incontinence, etc. There is a profound moral hazard in the state dishing out money for conditions such as these.