I have friends working these jobs. The consistent pattern is that they are only working the minimum number of shifts required to pay their bills, and they live pretty cheaply. You can't staff the restaurants when all of your employees only want to work 1-2 days a week. They could easily work more hours, and did before COVID, but are choosing not to now. And because restaurant work is extremely flexible and available on-demand, it makes their life flexible.
Many people have adapted their lifestyles to having less money and not working, especially with the generous unemployment benefits. Restaurant workers I know fall into that camp. My SO was receiving $75k/year on unemployment during COVID and turned down jobs paying $125k+ because they discovered the unemployment benefit was sufficient for a lifestyle of doing nothing that they were happy with. Not likely to take a job anytime soon either.
As a tangential observation, while I know many people in this position, none of them is doing much with their newfound free time. Just kind of coasting.
I don't think I've ever seen a sign advertising anything near that for restaurants.