> It's a natural consequence of capitalism

Adam smith was hostile towards monopolies (which your link refers to - nothing to do with price fixing although the conclusion is the same) and his critiques being very apparent throughout Wealth of Nations. This is a common opinion with proponents of free markets.

So what specifically about capitalism makes this a "natural consequence"?

The main feature of capitalism is not free markets, it's capital. And capital accumulates leading to inequality that only ever gets bigger unless we actively take steps against it as a society. Being against monopolies doesn't help if you're not willing to actually punish them.

Free markets are great but to really work they need to come with competition enforcement (including total M&A bans in high risk industries), strong unions, support for worker ownership and a tax structure that rewards productive investment instead of executive payouts. Market socialism instead of capitalism.