It's not a monopoly. Other phones and ecosystems exist.
On Windows I can buy a digital copy on the devs website and MS gets nothing.
On Steam, I could buy the game in another storefront (GOG), or Direct, and Valve would get nothing.
On Android I could use a separate AppStore and Google would get nothing…
In other words, in all those platforms devs can opt in to participate on a centralized store taking 30%, or keeping an alternative channel with higher revenues. On iOS, not so much.
Years ago, Apple had a loving relationship with Devs. Now Apple is an over demanding Karen yelling over her alimony payments every end of the month.
There is no way, on any major console, to "opt out" of the fee.
Mac has the same options available to developers as Windows.
Steam's third-party redemption mechanic is notable, and awesome.
On Android, different third-party stores have different fees. Samsung, for instance, charges the same 30% as Google. Epic's store charges 12%.
> It's not a monopoly.
I believe you have accurately described a cartel, however.
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.
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.
Steam also doesn't collect 30% for distribution (ie developer issued keys for things like bundles)