What stopping you from paying much more to these poor workers?
If you own a supermarket and want to pay a decent wage, bad news for you. Over long time you are likely to loose against your competitor who doesn't share your sentiment.
You may start with 100 companies of which 2 don't have any scruples and over time you will find out that these companies will take over the market along with other ones that will use this experience as learning on how to make successful business.
In the end the companies that are more aggressive win because that is just the nature of competition.
Governments are there exactly to protect individual people from aggressive predation.
If you can market this "decent wage" to your customers successfully enough that they buy in, then you can continue along this trajectory. (As a side note, most people here probably wouldn't take you up on that "decent wage" retail job, in part because they don't consider the wages very impressive. On the other hand, people who work at the "unscrupulous" grocery stores have agreed to those wages. So "decent" might have a sliding scale.)
As it is, it seems most people do not give a fuck about it and will still choose a store that is cheaper.
Actually, everybody knows Walmart or Amazon employees are paid shit and they still choose cheap.
That's were most economists, orthodox or heterodox (Neo-keynesian, MMT) agree, the capitalist class captured all productivity gains since the 70s. They disagree on the "how". And imho, you are right, they are not stealing wages, they are capturing wealth creation, it's not the same thing, and its pernicious because most of them don't understand how it works (and those who are able to try hard to not understand).
I used to think people were idiot to go after the banks ten years ago, but in fact, they were intuitively way more intelligent than a "rational" me. I just did not understood "how money works".
Their salary is a cost, like any other expense, and should be kept to a minimum, and only high enough to retain talented employees according to the market rates for those positions.
That's a fundamental principle of corporate accounting.