US Antitrust law generally is about pricing, collusion (over pricing or market access), and competition more so than just monopoly power. It is straight up not illegal to be a monopoly, only to abuse the position.

I'm surprised there haven't been more attempts at a "tying" argument against Apple's App Store and their platforms, but I'm also not a lawyer. It has what looks like a pretty clear, long history of being considered an anti-competitive practice by the courts. You can buy a Brother printer and not have to buy paper or toner from them, why should I have to buy my apps from Apple? And to be clear, that is precisely how Apple thinks of the relationship between the user and the app. Apple owns that relationship. They mediate. They manage. They facilitate. No one else. Users don't buy apps. Users pay Apple. Apple pays the app developers.