What you're describing looks more like the supermarket shelves. Some 80% of products in the cereal aisle are owned by three companies. Nobody really cares, but the switching costs to get a cereal outside that controlled space turn out to be pretty high (just try it with a family with kids).
If you want to make your own cereal, good luck; the supermarkets trust the Big Three and are pretty uninterested in flighting something new; shelf space is finite and people don't trust off-brand cereals.
The cereals care more about attention than price-competition. They know it's all the same crap; they want you to care more about whether there's a bear or a frog on the box.
And the same companies that make the cereals make several other verticals too, all carved similarly.
This configuration has not, generally, been considered illegal in terms of market regulation in the US. The standard is harm to consumers, not harm to non-incumbent manufacturers. Your battle to show why either of these spaces should be regulated more stringently is uphill against the default in the US to take a hands-off approach to market activity unless necessary to cure an obvious ill (and the ills here are non-obvious; how do we show the cereal market, or the software-services market, don't look the way they do because the incumbent players have hit on a locally-optimal approach to give value to customers, while customers are satisfied? Amazon, for example, are bastards, but they're bastards that have managed to unlock such efficient distribution and value-satisfaction for their customers that they rendered an entire ecosystem of competitors as obsolete as the buggy-whip manufacturer).