A few years ago, Ben Shapiro (like him or hate him) was speaking at a university, and a young Socialist stepped up to the microphone during a Q&A session to promote the related ideas of Unionization, Market Socialism (as opposed to political), and worker-owned businesses. The Socialist kept pressing the point that Capitalism is unfair to workers because workers provide all the labor for lower wages than business owners receive, which is classical Marxism at its best (or worst, as the case may be). The Socialist kept espousing what he thinks is "fair" with regard to wage gaps between the common worker and the CEO. Though to be clear, the young Socialist doesn't use the word "fair." Instead, he obscures that notion through a never-ending barrage of Socialist jargon. Nevertheless, that is his implication: Capitalism is "unfair" because workers do not earn as much as business owners.
However, Shapiro's answer provides one of the best explanations for why there is a logical reason for wage disparity, and I highly suggest that you watch it. There are two versions of this video: a shorter version (≈3:30) with just the highlights at https://youtu.be/3xq-q6a9tCM, and the full version (≈12:50) with the entire conversation at https://youtu.be/NUauABEnTZI. Unions are not a bad thing conceptually; the problem is that they often devolve into a bad thing in reality.
I have seen several situations where unions keep round-tripping back to the negotiating table again and again asking for benefits that are unnecessary (and I can cite examples), and the unions in these situations eventually force their parent businesses to become unprofitable (which is what has led to more robots on factory floors, self-checkout lines in supermarkets, jobs headed overseas, and self-service kiosks at fast-food restaurants). When unions demand higher wages for skills that are easy-to-train and therefore more or less expendable, businesses will do away with expendable laborers. (Either by outsourcing or automation, as I have just mentioned.) In a like manner, when unions demand benefits with significant costs from businesses like paid college tuition for workers' family members, six months of paid maternity leave, etc., these unions may force companies into bankruptcy. Don't get me wrong, paid college tuition and paid maternity leave are wonderful benefits to have, but when labor demands these types of things and profits cannot keep up with the increased expenditures, businesses are doomed to fail and everyone is out of work.
There was a greater need for unions a century ago, when modern labor laws had yet to be written and factories routinely abused their laborers. Times have changed, and everyone in the United States is in the top 1% of the world with regard to health, housing, employment, wages, etc. The quality of life in North America exceeds that of nearly the entire planet, and yet people still find room to complain - because people delude themselves into thinking that life is somehow "unfair" to them, which is typically because someone else has something that they want; either a higher salary, or a better job, a better house, a better car, a better spouse, etc. People want what they don't have, and that's what led to the "99%" riots a few years ago, and also to thousands of misguided Socialists and/or Unionists who decry their elevated global conditions as some form of suffering. Don't get me wrong, nothing is perfect and laborers still need representation to prevent potential abuse, but the irresponsible claims emanating from the mouths of most Unionists sound ludicrous when you take into account just how blessed some of the people doing the complaining really are.
As for Socialism, which typically goes hand in hand with Unionism and eventually devolves into Communism, let me take a moment to briefly editorialize. I have witnessed first-hand the many evils of Communism, which is why I cannot support anyone who is espousing either a Communist or Socialist system of government. While a logical argument can be made for socializing specific programs - such as healthcare - both Communism and Socialism are doomed to failure from their inception, because they seek to forcibly create "equality of outcome" instead of "equality of opportunity."
Everyone deserves an equal opportunity to qualify for the job they desire, and an equal opportunity to seek advancement within their respective places of employment. However, people deserve to be rewarded for their efforts, and if one person chooses to outperform their peers, they should be compensated for their efforts. Likewise, if someone chooses to accept greater responsibility within their company, or to step out as an entrepreneur and found their own company, they should also be compensated for their efforts. In a Capitalist society, they will be. However, in a Communist or Socialist society, everyone is forced into categories - often from which they cannot escape - and the outcome is dictated by the state. If Person A outperforms Person B, that doesn't matter; compensation must be the same for both people, for that is "equality" under Communism and Socialism. Of course, this system is untenable for those who are forced to suffer under it, which is why millions of people have fled Communist or Socialist societies over the past century, and millions more have been put to death or imprisoned when trying to protest their oppressive regimes. This is why most countries that adopt Communism or Socialism are forced to do so at gunpoint.
Most people who deride Capitalism do so because they have a misguided view of "fairness," which is generally an untrustworthy worldview. What is "fair" for Person A seldom seems "fair" to Person B if Person A has more than Person B, and therein lies the problem. In an equality of opportunity scenario, both Person A and Person B have the same chance to make more or less based on their personal participation, which is the ideal way of conducting society. However, in an equality of outcome scenario, if Person A has acquired more than Person B, that is forbidden by the state, and the government must intervene and force both persons to be "equal," which is not an ideal way to conduct a society.
This is what we have seen time and again with Communist or Socialist societies; they actively seek equality of outcome, and millions are forced to suffer as a result. The former Soviet Union, North Korea, China, the Warsaw Pact, and Cuba are just a few examples from the 20th century of what happens in an equality of outcome society. Even though barely a generation has gone by since the demise of the Soviet Union, there are thousands of gullible individuals who would gratefully embrace Communism and Socialism because they think that some part of their lives isn't "fair." These people are - as the Communists called them - "Useful Idiots." Such people hasten their own demise by failing to understand what they are promoting; as the old adage says, "Those who do not study history are doomed to repeat it."
I will now step down off my soapbox and back quietly away.