"Unskilled" is only unskilled because no proper training is provided. But you immediately notice if a cashier or cleaner is skilled or not. A cashier will know all the codes, all weird payment methods etc. And a cleaner needs to know the right tools for work, what chemicals to use and so on.
But if you block training and professional development in those jobs than yeah... they're unskilled and you have asshole justification for paying poverty wages.
I work in a "skilled" position where it is completely reasonable to expect to be able to hire someone and have to spend very little time training them. There will be a bit of onboarding as with any job, but the nuts and bolts of how to do their job.
I'm not saying "unskilled" jobs don't deserve a living wage, far from it. I'm just saying there's a reason there's a difference between the two, and one commands a premium.