First of All, we Lost the Vocabulary War

There almost no arguments in favor of the thing anti-capitalists dislike and call "capitalism." Every argument you see is an argument in favor of market systems and self-determination, which may be necessary components to some capitalist societies, but aren't important criteria to the people advocating against our current system.

It was a clever move on the part of whoever redefined, "capitalism" until it meant, basically, "all human, economic activity."

Fortunately, that's a double edged sword. One can advocate communism these days, calling it instead, "democratic workplaces", and "worker cooperatives", and "worker owned businesses."

And the overlords can't tell their workers, "that's communism!" Because they've spent the past 180 years redefining the terms.

Technically, according to their own warped-until-useless definitions, everything is capitalism. Because worker cooperatives ✨exchange goods and services for currency✨, they are capitalism too.

That said

Nevertheless, to answer the question: the most egregious argument in favor of capitalism is the businessman's story. You've probably heard some variant of it.

1. Tom has an idea!
2. He makes a prototype, refines the process, and then starts selling his whatchamacallit!
3. His watchamacallit has soared in popularity! The orders are coming in faster than he can meet them! He meets with investors who, in exchange for part of his company, give him enough startup capital to build a factory and hire wage workers.
4. He manages these wage workers, becoming gradually more wealthy as his business expands. He is now able to recline on a beach as the fruits of his labor continue pouring in. And those clever investors can recline on a beach as well!

By presenting (1) and (2) in the same narrative as (3) and (4), with the same characters, the story portrays (1) and (2) as mere extensions of (3) and (4), when in fact their presence in a society is hard to protect when (3) and (4) are also present.

The Wright Brothers never could get royalties from the people who used their inventions to build planes. Tesla never gained wealth from his society-altering technologies. Hedy Lamarr never earned a cent for inventing WiFi (according to some stories, she never wanted to, and donated her patents to the American war effort. But she still struggled with poverty in the middle of enriching numerous people).

The most telling part of this argument is that one could substitute (3) and (4) for literal slavery! And the narrative would look pretty much the same.

"Tom's getting more orders than he can handle, so he heads over to a slave auction to pick up some help! This way, he can produce more, and his new property can learn valuable skills! It's a win-win! This is how slaveowning helps Tom profit off of his idea!"

It was no accident that slaveowners of the American South considered their society and their struggle the last bastion against a communist takeover of the world.

It's a very compelling story (as misleading as it is). A tale where the beginning makes the listener root for the protagonist all the way to end -- even after he's committed unspeakable atrocities that plunder the world of its wealth.

Because we all want to be Tom and come up with a wonderful idea (or learn a rare skill) that makes us valuable to the world. We all want to bring some contribution to the table and have that contribution recognized.