The points say 'Can you walk 50m'? 'Can you put your hand on your head?' 'Can you pick up a pint of milk'. If you can do all those things but you're in constant pain or feel very poorly, you will probably score 'nil' points.
As somebody with reduced mobility and chronic pain, this simplistic question has never made sense to me at all. It's in most people's nature to push themselves to do what they are able to, often far beyond what they should. The question is 'Can you walk 50m unaided?' and that's that. No additions of 'on a good day' or 'without pain' etc. Somebody may turn up to their morning appointment on a good day and demonstrate that they can indeed walk 50m at a reasonable pace (albeit in a lot of pain), but doing it could well mean that their energy reserves for the rest of the day, or even the week, have been all used up. 'Can walk 50m' = 'is fully fit for work' isn't much use if it means you have to spend the next 8 hours in bed just to recover.
Thankfully for me, I'm not (yet) in the position that many utterly dependent people are, but reading these and many other heartbreaking stories send me the crystal clear message that, should I eventually end up in a highly-dependent physical condition, nobody in government could care less about me. They might rate theatres on how easy they make it for me to go and watch a play (which is a very good thing), but they wouldn't care about me having somewhere to live and the means to buy food and pay my bills, much less be able to buy a ticket for the play.
As PP have said, the big problem with many MH problems and learning disabilities/difficulties is that, by their very nature, many people who have them will not understand the extent or even the basic existence of their problem. Also, a lot won't appreciate how their over-optimistic determination and assertions will be used directly against them by these people who, we can all clearly see (however much they will deny it), have been specifically tasked to fight against the most vulnerable people in society and take away as much essential support as they possibly can. If it leads to a suicide then that's just another 'problem' off their books. I really don't see how these people can be human. Even Wonga would have rejected most of them for roles as aggressive salespeople for being too evil. They obviously have nothing to lose by rejecting people. Some of those who have family with the strength, knowledge, time, willpower and ability to fight will get the lies overturned, but the lying assessors will never be punished in any way. As for the rest of their victims, well we all know.
I have a relative with severe MH issues who will tell you things that are categorically not true and insist until she's blue in the face, and cannot understand why everybody disbelieves her. She is not deliberately lying, but she is telling 'her truth' as she genuinely believes it to be.
If somebody with severe MH problems insisted to these 'people' against all obvious signs of reality that they were Bill Gates (living in a HA flat in Wigan and eating beans on toast), I honestly wouldn't put it past them to not only check out Forbes magazine and rule that they therefore were very financially independent, but also to tip off HMRC that they were suspected of committing mass tax evasion.