Because the $20/hour price is artificial, so there aren't enough of them. If the price were not artificial it would balance until the number of jobs = number of seekers. But it won't, instead it becomes a lottery of sorts.

It also means far less job security for anyone who has such a job, because there are so many seekers employers will hold all the power, and will fire people easily. So it's also a high-stress job, but presumably worth it for that pay.

Which also answers your question: People will take lower paying jobs in order to have a bit more flexibility to request time off, or to make mistakes without fear.