Pure greed dictates that companies must do what is best for the individual company. Grow like cancer, engulf all competition, capture the market etc.

However, the owners of the company are humans that do not need to be constrained by those rules. Indeed, humans can hold other values higher - such as seeing that acting in the benefit of the entire society or even to benefit the entire planet is better than focusing on the best for a single corporation.

Pretend for a while that you are the majority owner of an internet company that preys on attention. You can capture the attention of millions, even billions of humans, and grow your vast empire. You can even become so successful that humans choose your product before the attention of other humans, choosing the dopamin kick from social media before real social interactions.

But...should you? Replacing the social interactions of an entire planet might make you rich today, but the price might be very high. Even your own offspring might suffer and perish in a world where social life has been replaced by hearts, thumbs up and endless scrolling.

It's unpopular in some circles to praise Bezos, but during this pandemic, I sure am glad that they had built such an expansive and performant logistics supply chain to help people get the goods they needed without having to go to the (often closed) malls.

That's a 2020 benefit to an entire society that arose from 25 years of effort to grow in a sustainable way.

It'd have been little different if there were a dozen of these companies, instead of a near-monopoly?

Is there a single market which has remained with a dozen of those companies which didn't have a cartel or preapportioned monopoly? Just with random variance I would expect three at most still around at the "traditional international size" level even if we started with a dozen Jeff Bezos Clones all told they were the real one with the goal of Amazon given identical starting funds (all too stubborn to merge).

The question is whether that moral owner can successfully compete in the unregulated marketplace. The answer is clearly No, which is why regulation is needed.