It follows from a few premises. The point of creating/allowing private companies to compete in a market and profit from doing so is to encourage them to innovate via competitive pressure. If you just wanted to produce well-understood goods or infrastructure then the most efficient way to do that is to pool resources and have the State do it, because they don't need to make a profit and, if not totally dysfunctional, are accountable to the people. If you let private companies consolidate power and influence then they largely escape competitive pressure and can streamline operations to maximize profits. That is, they benefit from the same efficiency the State does, but capture more of the value and remain unaccountable to the people, existing only to enrich their owners.

Many thanks for this level-headed response, I'll save that if you don't mind.

Yep. The issue with megacorps (and more generally monopolies) is that they want to have their cake and eat it too. You want capitalism but you also want to be the only one on the market. Pick a side.

They do pick a side: success at capitalism implies outcompeting your peers. Without regulation, there will be winners, and they will tend to be monopolies. Marx pointed this out in his book “Capital” (Das Kapital) in the late 1800s.

Capitalism without regulation can’t reach a stable equilibrium.

Free products are not the consequence of megacorps existence. Free products exist, because you are the product. Big companies also doesn't necessarily mean monopolies.