True true. I think when you see someone move to a new town because they can't get a job where they are, that's an example of some strange stuff going on.
In a premodern society you'd have the majority of people involved with the needs of basic survival: food-getting, shelter, defense. No specialists. A man might spend the day hunting and then spend some of the afternoon cutting wood and repairing his tools. In winter his efforts may turn to metallurgy, pouring copper into a clay crucible at the fire. Once food-getting technology improves so that you no longer need each person to be a food producer, and your population is large and stable enough that there is enough work for non-food-getters, specialists can emerge: one man spends all of his time making tools, trading them to others for food. Or spend his time making shelters and fences.
As specialists emerge their crafts become complex enough that subdivisions appear. At first the smith specializes in making swords. Then he realizes he can make high-quality blades while a less-skilled or perhaps just differently-specialized worker makes the accessories.
At some point you will spot a problem as a tool-maker; your town might already have one, and no more are needed. You need to go where there are people who want to buy your tools. This isn't going to happen until there is a high level of tool ownership or of tool production: if everyone has tools, they don't need many tool-makers, and if there are a lot of tool-makers, there will not be enough local demand to purchase all of those tools. So if a person wants to sell his labor, he needs to find a buyer of labor.
If you're in a pre-modern culture I find it hard to believe that there is such a richness in food, tools, services, that people have no further needs. Yet that's the case today: you can have a town with a dairy, for example, and the dairy has employed all of the people they need. The town store has enough staff. The sheriff has his deputy. A new person entering represents a tiny amount of each need: a need for more food from the dairy, goods from the shop, security from the sheriff. But the existing people can easily expand their production to fulfill his needs. But his labor is unneeded.
I think huge towns work the same way. It's just that there are constant vacancies, overproductions, understocking, confusion, miscommunication. You never see exactly what the whole system looks like because it's so complex. And it can absorb individuals easily as long as there isn't a large-scale shift. Such as many employers laying off workers.
You might, for example, see a company of 100 employees respond to financial difficulty by laying off workers. They can control labor costs, but they can't just reduce their rent or insurance or materials cost. All they can do is sell more or spend less in payroll. And if they're in this situation because they are selling less, they must reduce payroll. If their sales are down 10%, they can probably lay off 20% of their workforce and reduce production by 10% - working the remainder harder to cover the difference in man-hours. These people are afraid they'll lose their jobs, so they're willing to work harder. And if they don't, and quit or are fired, there are 20 workers sitting around skilled on those machines who might be willing to return.
If every company does this, you end up with less production next quarter, fewer sales, but nearly the same profit. But you've also reduced your market by 20%, of the people who are now so strapped for cash they can't afford your lattes and lawn furniture, your ceramic giraffes and frilly pillows. Which means next month, your sales decline even more.
This reverses the normal trend for a business: you begin small, with little labor and little sales, and gradually increase your sales as your business becomes popular. As sales increase, you must increase production, and so must add more labor. Downsizing is bad for the employees, but it's a slow death for the business unless things turn around.
I'm actually very interested in this slum economy, of the 20% who get laid off and try to find work but can't, and no longer have money but must survive somehow. Are we avoiding the emergence of a slum economy through welfare programs like housing, utility, and food assistance? If so, will enrichment of those programs be an effective transition into a post-scarcity culture?