Why aren't some people signing on?

There's supposedly millions of working aged adults who are not employed and are "not looking for employment", I don't really understand that.

Sure, you have to fire off a few applications and rock up to the job center once a week but it's worth it for the little bit of job seekers allowance you get? Every little bit helps right?

Does anyone know one of these kinds of people? I imagine a large portion of that is young people living with their parents who are basically just paying their way.