You really are failing to understand the nature of the problem. Living with a disability, there is no way to predict ability to do a task. It might happen tomorrow as planned, or never. You can't earn a living that way. Since you obviously won't read the links I posted, let me repost a little bit of it here.
As regular readers will know, I have a variable illness. There are days when I can walk to the shops. There are days when I can’t get out of bed or as far as the bathroom. There are times when I go out without using a walking stick but I have to use one to get home. (Staggering all the way.) There are times when I am well enough to ride a motorbike. There are occasions when I have ridden that motorbike for four hours, but then an observer has no idea what I go through after doing that. If someone sees me walk to the shops without a stick, they can’t make any assumptions about what I can do the next day, or even an hour later.
A major flaw in public thinking, and in the Work Capability Assessment, is the idea that if a person can do something once, they can do it again. I can choose to do a task at the expense of a day, a week, or however long in bed. I couldn’t do that task every day. I certainly couldn’t do it all day every day. I can sign my name, but I can’t write a page of text. I can commute to work, once, but couldn’t get home again. I could set up a whole website in a couple of hours in the middle of the night, but I couldn’t do it all day every day for a living, or tell you when I could next do it. Other people with health problems might be able to mow their lawn, put up a shelf or paint a wall, but at a similar cost to their health along with the associated recovery time. Living on benefits, as many of these people do, they are poor. They can’t afford to pay someone to do these jobs necessary in everyday life, and so they do them themselves, and pay the health costs later. Yet neighbours and passers-by see them doing these tasks, and instead of asking “can I help?” they phone the DWP and report them as a benefit fraudster.