I believe you're arguing in bad faith. Are you just becoming aware of these companies 30% cuts? Why were you calling Apple a monopoly if you think this is a cartel issue? What evidence do you have of cartel behavior other than that people followed suite in setting rates?
You shouldn't make that assumption without significantly more proof. Why can't it be both? In the App space Apple has the dominant position in a duopoly, and in the software distribution space more broadly there is cartel-like behavior occurring regarding commissions because distribution has become a major chokepoint (I'm guessing thanks to DRM, but that's probably oversimplifying).
There are absolutely antitrust issues with grocery stores - Kroger/Albertsons is being contested right now! It's a major contributor to food deserts. It's not a cartel because the main players aren't acting in concert, but they are attempting to monopolize regionally. Walmart has also concentrated way too much power, but that's the 800 lb gorilla no one wants to touch.
That said, gas stations and grocery stores deal in commodity products. The video game companies are much more integrated with many games that will only run on one system, stores will only run on one type of hardware, and games are not transferrable between competitor systems. Competition is extremely restrained because of these exclusive and limited distribution agreements. The fact that they all maintain the same line on prices is a very clear signal of cartel behavior when you take the context into account - they are using control of a distribution chokepoint to control competition and pricing power. That's the essence of why antitrust laws passed in the first place.
Because developers go where the users are. That's all that matters.
Epic had to pay developers, a lot, to bootstrap the Epic store so that there were games there that mattered. And even now, with a decent selection, most PC gamers still use Steam. Because all their games are on Steam already. And every game they care about is on Steam.
So Steam can charge 30% and Epic can charge 12% and nothing really changes.
Probably has to do with market power. For my money, none of these companies should be allowed to own a game marketplace. The distribution channels should be stand alone from the hardware and software producers.