But in the case of Microsoft, Nintendo and PlayStation those aren't (necessarily) the only costs of selling. If I choose to, I might decide to exclusively distribute through Amazon, who will take their own cut of the sale price (or cost price depending on the arrangement). Or I might decide to distribute through Walmart as well at a different cost to me. In all cases I have to pay Microsoft(or Nintendo or Sony) but I get to decide, overall that I make more money selling it on Amazon and paying them than selling it myself.

What this boils down to is, what my options are when selling my game. On Xbox, Nintendo Switch and PlayStation I have choice over how I sell my game and can choose different avenues that have different pros and cons (More sales with Amazon, but less profit per copy sold vs. More profit per copy, but less sales when sold direct). With iOS you don't have that choice, hence they are a single buyer.