Here's a few examples:

- High implicit labour prices. When you add up all the required costs to hire somebody (minimum wage, healthcare costs, payroll taxes, etc.) it's no surprise that there's going to be some portion of the population which can find no work at that price.

- Unnaturally high costs in the most productive areas. Cities with lots of jobs will naturally have high costs, but a lot of regulations, like zoning law and building codes essentially put a high minimum price on living in particular places. This makes it very hard for people to move to the work.

- Licensing and accreditation. There's tons of jobs (many of them service-sector jobs) that require a license, lots of permits, and often particular kinds of schooling to be able to legally do them.