@Foddplqf labour markets are often not competitive. They're largely characterised by local monopsony power, like when an Amazon warehouse or supermarket is the only employer in town for someone who doesn't have a specific, specialised skill set.
For salaried jobs, work and pay can also be quite insulated from the actual labour market - if you're asked to stay an extra hour to finish a task, or to have a security check before you're allowed to leave a retail job, or a chat with a manager about not smiling enough at customers, for example, even if these things would mean your hourly wage fell below minimum wage or just below the wage you actually seem acceptable, you are simply not operating in a market - you're contractually obliged to do whatever your employer tells you too, or you can quit, which is costly and time consuming - a functioning free market needs alternatives, and at that moment in time you don't have any.
The growth of "flexible" employment, where you're not guaranteed any hours, also massively tilts the employment relationship in favour of employers. If you complain or don't stay late or don't do a task that's dangerous or that you didn't agree to do when you got the job, you can just not be given any hours next week.
For all of these reasons, jobs that people don't really want to do can still be badly paid.