I think this answer misses the mark a little bit with regards to the context of what it is about capitalism that causes so much controversy.
People who critique capitalism aren’t usually advocating for an economic system completely devoid of private ownership (though some are). They’re often raising issue with a certain type of capital ownership.
Say you’re the owner of a sprocket manufacturing corporation, and I’m a worker. You yourself don’t work, you just inherited a sprocket empire from your grandfather, who founded that sprocket empire using funds from selling his emerald mines that were worked by slaves.
I put in an honest days work 5 days a week, and in those 5 days, I produce $2000 worth of sprockets. It costs $10 per week to maintain the sprocket machines, $10 per week for electrical power to cover my sprocket making activities, $30 per week to repay the loan that was taken to build the factory, and $50 per week in other miscellaneous expenses needed to allow me to make sprockets.
That means that of the $2000 worth of sprockets I produced, $1900 of profit was generated.
You pay me a salary of $500 per week, and collect $1400 per week from my (and each other laborer’s) work.
The point of criticism is that you’re accumulating wealth, which other people had to work to produce, without doing any work yourself. You’re simply a parasitic freeloader on society because of an arbitrary concept of “ownership” over something that you don’t use personally.
Some responses to these criticisms include the following:
“You should just negotiate for a higher salary, or go work somewhere that’s paying more.”
This is the example from classical economic theory, and there are a whole handful of reasons that it doesn’t work. The voluntary exchange between a worker and a capitalist (meaning one who owns the means of production) isn’t actually (fully) voluntary. A worker who finds his working conditions unsatisfactory can’t reasonably just choose not to work. The threat of financial ruin, homelessness, and starvation act as a metaphorical gun to the head of the laborer giving the capitalist a significant negotiating advantage.
Add to this the fact that it’s been theorized and demonstrated that capitalism tends toward regulatory capture and monopoly, and you have a situation where the means of production become more and more concentrated in the hands of a group of elites, while the workers’ bargenaining power becomes weaker and weaker due to less competition in the labor market.
“The capitalist deserves the profits that they extract because of [this work that they’ve done]”
If the owner of the factory is functioning as a floor manager, they should be paid a fair salary for being a floor manager. If they’re working as a director, they are entitled to a director’s salary. These critiques of capitalism aren’t saying that there should be no hierarchy in an enterprise (though some alternatives to capitalism do call for that). Just that the only people who are entitled to the wealth from something that’s produced are those who are working to produce it.
This same thing goes for other forms of capital ownership too, by the way. Landlords are a classical example. I’ve heard it claimed that landlords are entitled to their rents because many of them work so hard at repairs and managing their properties. They’re totally entitled to be compensated for any labor they engage in, but the wealth that they extract from tenants far exceeds the value of the labor that they supply. Which is kind of the whole point of rental property, if “investors” couldn’t extract a passive income (income in excess of work performed), they wouldn’t be buying homes and then renting them out.