Since we've had periods of almost full employment, it seems like almost everyone agrees to work if the job offers are good enough.
You are confused. Full employment is not defined as "everyone works". Full employment is defined as "everyone who's looking for work can find it". People who are not looking for work are not counted as unemployed.
For example, at the moment the US unemployment rate is 4.7%. But the employment-population ratio is only a bit above 60%. So 40% of the US population between 15 and 64 does not work. But the unemployment rate is below 5%.
legally define what constitutes a "reasonable" job for a given person
That's called "minimum wage", isn't it?
If the market is good at providing reasonable jobs
Nope.
The market is good at creating value and allocating resources in such a way as to maximize value produced. A job is a cost. You should prefer a lot of value and few jobs. That's what high productivity of labour means.
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