There is a spectrum between inability and unwillingness. On one extreme end, imagine someone who is omnipotent and omniscient, could do literally any job in existence flawlessly, but absolutely refuses to do any job, and then on the other extreme end, imagine a person who would be willing to work his ass off doing literally any job in the world for any amount of pay, but happens to be an unconscious quadriplegic. The former is 100% able but 100% unwilling, the latter 100% willing but 100% unable.
Most people fall nowhere near either of these extremes. Most people are physically able to do countless types of jobs, by mere virtue of their ability to sit in a chair and use their hands on a keyboard. Physical ability to work is not usually the problem, because ever increasing numbers of jobs do not require significantly special physical abilities. Increasingly it is the cognitive/intellectual/mental/emotional mix of abilities, plus experience and credentials, that makes a person not only "able to work" but also desired by employers. The problem is there will always be millions of people at any given time that no one in the economy wants to employ. Even if they're theoretically able to work, theoretically able to learn/be trained, hell even if they're willing to work, that doesn't mean any employer out there, public or private, necessarily has any need or desire for them. And this problem will never go away. If we have several million working age people the product of whose labor the society doesn't want, why are you trying to compel society to buy their labor anyway?
Your basic needs will be met if you refuse or are unable to meet them yourself, yes. However those needs will be met in a bare minimum way such that, were you to be completely satisfied with that existence, it would indicate things about your personality or mental state that shed light on why employers probably don't want you to work for them either. Let me put it another way: when it comes to people who would prefer to be lazy and have no interest in their organization's mission or what their work entails, the feeling on the employer's side of the table is mutual... just stay home, I'd rather not deal with you.
As soon as you take this bunch of people whose personalities/mental states/bodies are incompatible with the modern work environment and require them to start doing jobs, that means you 1) have to make up tasks for them that no one else is already doing, 2) those tasks are likely to be make-work in nature (because everyone else is already doing the stuff that matters), and 3) it means you need to hire other people to competently babysit them in their performance of these bull**** tasks, which is like appointing a calculus and advanced algebra teacher to teach remedial math to a hodge podge bunch of delinquents during detention.
I don't disagree with conservatives very strongly on very many things, but this is one of the biggies. Workfare is a very foolish inconsistency in the conservative ideology. Conservatives want to be pro-business. Forcing private sector employers to hire people society as a whole would rather not hire is not pro-business. Conservatives want government to be more efficient. Forcing government to employ the very people government has already decided it doesn't want or need to employ works directly against that. There is nothing about workfare that is aligned with conservative ideals.