There are loads and loads of chronic conditions that have no clinically objective test, and loads of websites that describe these in detail and state exactly how to describe the impact such a condition would have on your life in a way that meets maximum criteria for pip claims.
Examples:
Fibromyalgia
Hypermobile ehlers danlos
ME/chronic fatigue syndrome
Anxiety and depressive disorders
There are people who are truly, severely impaired by all of these (lets call them Group A)
There are people who don't have easy lives, who struggle, who truly truly believe they have and are severely impaired by all of these. Some of these people could probably also be helped by other things, like social support, fulfilling work & hobbies, and healthy lifestyles. (Group B).
There are people who kinda know on some level they probably aren't really that ill, but meet some criteria for these conditions, and have lives that are really hard (single parenting, young kids, caring responsibilities, low income, lack of social support networks), and want to claim benefits to make life easier. They see other people doing it, and life isn't fair, and they just want a break. (Group C).
There are people who simply see an opportunity and are willing to put in a massive effort to access benefits they know full well they don't deserve. (Group D)
A lot of people are A.
But pretending there arent also plenty of Bs, Cs and Ds is just naive. Modern western capitalism & social structure has really left a hell of a lot of people behind. Our economy has floundered in most of the country for decades. The NHS has suffered from strain & chronic underinvestment. Our society is unequal and unfair.
People are tired, poor, and lacking hope. Is it it any wonder people want benefits?