You see where this is going. If Fred really was underpaid relative to the value he produces, why isn't he getting a better offer elsewhere? You could postulate there are lots of Freds bidding for his job, keeping wages low. But then were are the new companies being created for the simple purpose of hiring boatloads of Freds and making 10x their pay?
Being underpaid seems like an unstable state in a free market.
Someone taking $10/hr for $100/hr creation in value in order to get the job over the guy who offered $11 is what is actually happening.
Often when the greedy capitalist loses, everybody who works for him also lose. People talk about the risks capitalists take and try to compare them to workers using sheer dollar values. That valuation completely sidesteps the diminishing marginal utility of cash.
If a capitalist goes out of business and his $10 million factory ends up getting sold off for a net of $3 million after paying his debts, we can say he lost at least $7 million. His workers, on the other hand, don't lose anything but their jobs. However, those workers might have been living from paycheck to paycheck, leaving them far worse off than the capitalist who still has $3 million in his pocket.
Why would an owner do that?
Bob, Ted and you convene secretly and agree you won't poach each others employees or pay them more than $15/hr? And since you're the only game in town and can afford to undercut any competition until they're run off, everyone's forced to deal with it.
For example, suppose I have a machine that, when a lever is pulled, it manufactures a gold nugget. A worker is needed to pull that lever - but is the bulk of the value being created by the worker or the machine?
It gets down to how one does the accounting for productivity. Get it too far wrong and one is likely to go out of business.
Unions have sometimes priced themselves out of the market, meaning the business closed. Didn't that happen with Hostess?
Not everyone has good information about this market. I don't get to see how much someone paid for Fred's labor. That means someone is likely to underbid for my labor if I'm in the same business, to which I must take time to convince them otherwise. I can also try to get them to overbid for my labor, to which they must convince me otherwise. And off we go to the negotiation. Why is it good that we have different people producing similar levels of output and are paid differently?
As a Fred, assuming you want to continue to be an employee, you should sell your labor to the employer who gives you the best overall deal for you. As an employer of Freds, you should seek to get the best mix of Freds you can for the lowest sustainable rate. Bringing in other actors to arbitrate and equalize pay doesn't add anything that the actors in the system can't already do, IMO.
(Maybe Fred1 values that his employer allows dogs, or has bike parking, or is a short walk from his house. Maybe Fred2 likes his employer's stance on social issues, or benefits package, or likes working 4-10s instead of 5-8s. How will the central pay authority value those things better than independent actors making their own choices?)
In the past they did create wealth, by actually founding those successful businesses. Of course that might have been luck and markets aren't perfectly fair, so maybe they don't deserve 100% of the profits. But still they did create something of value, at one point in time. A thing which continues to produce value today. It's not like Walmart just materialized out of the aether and Sam Walton just happened to find it first.
Of course deontological arguments are silly and it's impossible to know who "deserves" what. I think a consequentialist argument is much stronger, that taxing rich people and redistributing the money to everyone, creates more benefit to society than it harms.