Why wouldn't other, competing corporations just... not raise prices and increase their market share?
This lack of real choice is hidden behind a plethora of brands all owned by the same large corporations.
And it turns out when there are like 4 large corporations that are responsible for almost everything produced in a market it's really easy to do de facto price fixing (in the sense that they [usually] aren't officially talking to each other but have unspoken ongoing gentleman's agreements) allowing all parties to get a nice share of the price gouging with no party triggering a race to the bottom.
If you're going to ignore the obvious problems with massive consolidation in areas like supermarket chains and virtually everything sold in those chains or the massive problems with consolidated telecomm/ISP companies and the de facto regional monopolies they carve up then I don't think you are arguing in good faith.
Kimberly-Clark, Proctor & Gamble, Edgewell, and Energizer comprise like 90% of the pads and tampons you'll find on the shelves in an American grocery or discount store.
The largest shareholders of all of the above are the same: Vanguard, BlackRock, and State Street. Combined, they make up ~25% ownership of all of them.
Alternatively: firearms
For a while, Cerberus Capital Management owned approximately a majority of US firearms companies by production, including: Remington, Barnes Bullets, Bushmaster, DPMS, Advanced Armament, Marlin Firearms, H & R Firearms, Para USA, The Parker Gun, Dakota Arms, Tapco, and Storm Lake Barrels. All of the above were acquired by Cerberus from 2006-2009.
All of the above were folded into "Freedom Group". That was later rebranded as "Remington Outdoor Company", then sold piecemeal three years later (in 2020).
As a CEO, why would sharing 25% of shareholders with another corporation stop you from exploiting their weakness, increasing your market share, increasing your stock price and in the end, increasing your bonus? You're a publicly traded corporation, your contract is public, your bonus mechanic is public, these 25% don't have any other means of control over you.
Having them as a shareholder in common just means they are in the same index and has no impact on potential collusion.
What keeps prices under control in any market is that somebody with supply capacity loses out and doesn't use that supply capacity - because the price achievable doesn't make it worth using that supply capacity.
That's the issue we have at present. There is insufficient spare supply capacity available to be brought online if prices go up. The solution is shifting consumption to investment, but at present the short term view is seen as more lucrative than the long term one.
It's a great question. Usually that competition is what I would expect, but I wonder if there are really the proper incentives in place for competing corporations to compete. I've speculated about this a few times in the past, but corporations ultimately are accountable to their shareholders. And it's increasingly common for their shareholders to also own shares of all their competitors, which is the whole idea of buying an index.
So, what is on one hand necessary and inelastic can also be a luxury.
Even things called 'staples' are flexible, you won't die of malnutrition if you don't eat eggs.
Which is exactly my point: either a rival corporation would be able to get market share via undercutting, or price increases are not actually that sensitive to consumer to begin with.