That doesnt make sense to me.

Seems like theres either a demand for something or there isnt.

And the amount of demand sets the rate of pay.

The invisible hand.

Demand is a function of price. At any given price there is a quantity demanded. To know the price you also need to know the supply function. There is a demand for socks but if no one is will to supply socks for less a million dollars, the quantity demanded of socks at that price could be 0.

In most markets seems like commodity level items and services like baggers and socks would have plenty of supply.

In a free market, commodity-level jobs will only happen at commodity-level wages. But it's not a free market, because of minimum wage laws. If you set the minimum wage too high, the number of baggers may go to zero, not because of supply but because of demand - the price is higher than any grocery store is willing to pay.

You can argue about the need for the minimum wage laws. You can argue about the morality of paying a living wage. But that's a different argument.

The demand curve might not be gradual, there might be demand for e.g. 1 million jobs at $15 but 100 at $16

In some countries there's something called a "minimum wage" that sets the minimum value of a job that is allowed to exist, and any job that isn't worth that amount ceases to exist, or has fewer people doing it, or has partial or full automation doing it.

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There’s a demand, but the price we want to pay for “grocery baggers” is usually less than minimum wage, so the job usually can’t (legally) exist. The cost (wage) is fixed higher than the value added.